What is the difference between fervor and ardor?

Take a look at the definitions in the Cambridge online dictionary. The definitions are very similar but the examples start to show the difference:

Ardour: Definition-

great enthusiasm or love:

Example-

His ardour for her cooled after only a few weeks.

Fervour: Definition-

Strong and sincere beliefs

Examples-

The country was swept by patriotic fervour.

nationalist/religious fervour

The difference is that "Ardour" is a personal emotion, pure and simple. It is the result of being swept up by an emotional response to a stimulus, very often a sexual one, while "Fervour" is an emotion resulting from a more intellectual conviction or a social pressure.

One feels ardour for someone to whom one is sexually attracted or for works of art, literature, music and so on and, although one might extol the producer to friends and acquaintances ("I think Hockney's marvellous", "Isn't Ed Sheeran wonderful?") there is no consistent attempt to convert them to one's own way of thinking.

Fervour, on the other hand, almost always relates to a political or religious conviction or a commitment to a community, state or people. For instance, one might be driven by nationalist fervour, particularly in times of war or crisis, or one might be fervent in one's commitment to protecting the environment. People with fervour for a subject usually try to convert other people to their way of thinking.

It is possible to be driven by both ardour for a particular object of affection and by fervour to protect it. For example, many environmental campaigners have been driven by an emotional response amounting to ardour for a particular landscape, animal or plant to wage a fervent campaign to protect it.

Primatologist and conservationist Dian Fossey is a good example of this. Her commitment to mountain gorillas was almost certainly driven by the ardour of her affection for them but the fervour of her campaign to protect them led both to a greater degree of protection for them and, probably, to her murder.

In the circumstances when the difference is subtle it might help to consult multiple dictionaries and examine as many usage examples as possible (or, rather, study usage examples until it becomes clear how the two words differ).

The definitions from the Cambridge dictionary are already discussed (in one of the other answers posted here). I will cite Merriam-Webster, Macmillan, the OED and, in P.S., Longman and Collins; they offer definitions that are somewhat different from those offered in the Cambridge dictionary.

Different dictionaries use different methodologies: some attempt to document the usage observed throughout history, others aim to capture how the word is used now.

When different dictionaries offer different definitions, these definitions could be understood as records of the observations of word usage noted. For this reason (and as there is also a potential for an editorial error), the definitions cannot be taken as final judgements on what the word means and require extensive and detailed substantiation through usage examples.

The question of whether a particular instance of historical usage is still viable today despite it appears to be no longer observed in the present may be impossible to answer. Notions and meanings that they embody may not cease to exist when a particular group of people looses an awareness of them. For practical purposes, it makes sense to limit this discussion by saying that writers, readers and scholars may make their own decisions as to whether any particular instance of usage holds a significance for them and is suitable for what they are seeking to express.

Here is Merriam-Webster on ardour (ardor) and fervour (fervor):

Ardor:

1 a: an often restless OR transitory warmth of feeling (the sudden ardors of youth) b: extreme vigor or energy : INTENSITY (the ardor of a true believer) c: ZEAL d: LOYALTY 2: sexual excitement

Fervor:

1: intensity of feeling or expression (booing and cheering with almost equal fervor — Alan Rich; revolutionary fervor) 2: intense heat

Merriam-Webster also notes

FERVOR implies a warm and steady emotion. (read the poem aloud with great fervor) ARDOR suggests warm and excited feeling likely to be fitful or short-lived.

However, this difference does not appear to be noted elsewhere (see P.S.)

Macmillan gives these definitions:

Ardour:

1 very strong feelings of admiration or determination 2 LITERARY very strong feelings of love

Fervour:

very strong feeling or enthusiasm

To draw a separating line between these two words on the basis of these particular definitions, we could say that even though these words overlap in meaning and both refer to an intensive feeling or to an intensity of feeling, 'ardour' may have a stronger component of being a reference to the feeling itself and contain a sense of eagerness, love, loyalty, while 'fervour' is more of a reference to the intensity of the emotion.

Here is the etymology of the two words, according to Merriam-Webster:

Middle English ardour, borrowed from Anglo-French ardur, ardour "burning, fever, passion," borrowed from Latin ardōr-, ardor "burning, flash of light, extreme heat, mental excitement, eagerness, passion," from ardēre "to burn, be fiercely hot, be violently excited, be eager"

• -ōr-, -or (earlier *-ōs-, *-ōs), abstract noun suffix ARDENT

Middle English, from Anglo-French & Latin; Anglo-French, from Latin fervent-, fervens, present participle of fervēre to boil, froth — more at BARM (Middle English berme, from Old English beorma; akin to Middle Low German berm yeast, Latin fermentum yeast, fervēre to boil, Old Irish berbaid he boils)

It appears, ‘ardour’ is based on ‘to burn, to be violently excited, to be eager’; ‘fervour’ stems from ‘to boil, to froth’. While violent excitement may be akin to either burning or boiling, the sense of eagerness and/or desire present in ‘ardour’, is a somewhat different idea. It seems, the etymology suggests a similar separating line to that drawn above.

From the point of view of the OED:

Etymology:

< Old French and Anglo-Norman ardour, earlier Old French ardor, -ur, modern ardeur < Latin ardōr-em heat, < ardēre to burn. The spelling ardor, assimilated to Latin, has been in use since 16th cent.

Middle English fervor , -our , < Old French fervor, -our (modern French ferveur ) = Provençal fervor , Spanish fervor , Italian fervore , < Latin fervōre-m , < fervēre to be hot.

Note: Merriam Webster interprets fervēre as ‘to boil, froth’, while the OED - as ‘to be hot’. Latin dictionaries include both meanings.

(Multiple sources on Latin exist and can be consulted, and here is, for instance, a Latin-English Dictionary “for the use of schools” of 1873 Latin)

The OED shows that

ardour/ardor is used in three senses:

1. Fierce or burning heat; concrete fire, flame. 1670 C. Cotton tr. G. Girard Hist. Life Duke of Espernon ii. viii. 409 To qualifie the excessive ardours of the Sun. 1755 B. Martin Mag. Arts & Sci. 103
   A Degree of Ardour equal to that at the Comet. 1814 H. F. Cary tr. Dante Vision I. xxvi. 113 Within these ardours are the spirits, each Swath'd in confining fire.

2. poetic. An effulgent spirit. (this sense is obsolete) 1667 J. Milton Paradise Lost v. 249 The winged Saint..from among Thousand Celestial Ardors..up springing light.

3. figurative. Heat of passion or desire, vehemence, ardent desire; warmth of emotion, zeal, fervour, eagerness, enthusiasm. Const. for. (The earliest sense in English: formerly used of evil passions, but now only of generous or noble impulses.) 1678 A. Marvell Acct. Growth Popery in Wks. (1875) IV. 313 This dispute was raised to a greater ardure and contention than ever. 1756 E. Burke Vindic. Nat. Society 8 And feel such refreshing Airs of Liberty, as daily raise our Ardor for more. 1769 W. Robertson Hist. Charles V III. ix. 139 Hurried on by a martial ardor. 1819 J. Q. Adams in C. Davies Metr. Syst. iii. 131 Inquiries..pursued with ardor and perseverance. 1842 J. Wilson Recreations Christopher North II. 278 The bright ardours of boyhood.

fervour/fervor has two senses:

1. Glowing condition, intense heat; in reference to water, boiling, seething (the latter is obsolete) 1725 A. Pope tr. Homer Odyssey III. x. 184 Some pow'r divine..Sent a tall stag..To cool his fervor in the crystal flood. 1794 H. L. Piozzi Brit. Synonymy I. 207
   Such effects follow naturally the fervour of an African climate. 1813 P. B. Shelley Queen Mab viii. 102 Those deserts..whose..fervors scarce allowed A bird to live. 1891 Sir R. Ball in Argus (Melbourne) 16 May The moon was also doubtless in a condition of equal fervour.

2. Warmth or glow of feeling, passion, vehemence, intense zeal; an instance of the same. 1340 R. Rolle Pricke of Conscience 250
   Fervor of thoght. ▸ c1384 Bible (Wycliffite, E.V.) (Douce 369(2)) (1850) John ii. 17 The feruour of loue of thin hous hath etun me. 1729 W. Law Serious Call xiv. 240 And begin to know what saints..have meant, by fervours of devotion. 1830 I. D'Israeli Comm. Life Charles I III. ix. 196 The fervour of loyalty vied with the pride of magnificence. 1882 A. W. Ward Dickens iii. 50 A fervour unique even in the history of American enthusiasms.

The separating line drawn earlier is also seen in the OED, at least to some extent.

Merriam-Webster’s usage examples for ardor and fervor, drawn from sections ‘examples in a sentence’ and ‘on the web’:

Note: in capitals, after each example, I include a substitute word/s that, in my view, express the meaning of ‘ardor/fervor’ in the sentence

ARDOR

the sudden ardors of youth

candidates for citizenship reciting the oath of allegiance to the United States with all the ardor that they could muster PASSION

Wind and solar power did not begin to expand dramatically because of a newfound ardor for the environment, the demands of climate change, or cheaper technology. PASSION — Doug Struck, The Christian Science Monitor, "Power pivot: What happens in states where wind dethrones King Coal?," 21 Aug. 2020

The overwhelming humidity of the Chinese summer was not enough to stifle the ardor of the crowds of 20-somethings honoring Mao Zedong, the founding father of Communist China. PASSION — Anna Fifield, Washington Post, "Trump views China’s Communist Party as a threat. Young Chinese see it as a ticket to a better future.," 4 Aug. 2020

That, combined with the rise of a new generation of white Republicans more interested in big business than racial equality, cooled GOP ardor for Black civil rights. PASSION, EAGERNESS, ZEAL — Brian Lyman, USA TODAY, "Fact check: Yes, historians do teach that first Black members of Congress were Republicans," 19 June 2020

The same scene played out over several days last week in Cairo, where the minor matter of a global coronavirus pandemic has failed to quell couples’ ardor. PASSIONS, DESIRES — Declan Walsh, BostonGlobe.com, "Tux, gown, masks: Arab couples scramble to marry during a pandemic," 25 Apr. 2020

Here, perhaps, was one source of the ardor for universal justice, and the loathing for political reaction, that would sustain Marx throughout his life. PASSION, ZEAL — Peter E. Gordon, The New Republic, "Karl Marx’s Prophetic Longing," 6 Apr. 2020

Instagram, in particular, is home to some of the most creative expressions of VHS ardor. PASSION for — NBC News, "VHS tapes are back in vogue as everything old is new again," 7 Mar. 2020

Esther is in love with literally the boy next door, John (Tom Drake), who is too clueless to comprehend her coy ardor. (PASSION) — Peter Rainer, The Christian Science Monitor, "Home theater: Escape into the joyous world of musicals," 22 Apr. 2020

FERVOR

As Nina has grown more observant, Andras has become distanced from her. Her religious fervor doesn't interest him. Coming to tradition late, Nina has all the pedantry of an autodidact. Her strivings seem inauthentic to Andras, and not at all spiritual. PASSION, THE INTENSITY OF EMOTION — Allegra Goodman, Kaaterskill Falls, 1998

In her renewed fervor, Norma fears that the past decade has turned women inward, away from one another, and away, too, from the notion that solidarity among women is ultimately a source of personal strength. PASSION — Anita Shreve, New York Times Magazine, 6 July 1986

Reciting, her voice took on resonance and firmness, it rang with the old fervor, with ferocity even. PASSION — Eudora Welty, One Writer's Beginnings, 1983

The fervor surrounding her campaign continued right through election day. PASSION

The novel captures the revolutionary fervor of the period. PASSIONS

In the summer of 2013, royal baby watch hit a fervor as the world waited for Prince George. PASSION, THE INTENSITY OF EMOTION — Elise Taylor, Vogue, "Queen Elizabeth’s 11 Best Witty One-Liners," 10 Sep. 2020

Led by the anti-ICC fervor of John Bolton – who rose to become ambassador to the United Nations – the George W. Bush administration initially took steps to weaken and threaten the court. PASSION — Howard Lafranchi, The Christian Science Monitor, "Justice for all? Behind US targeting of international court.," 9 Sep. 2020

The thrill of March Madness and the fervor of the Final Four have Jernstedt’s fingerprints all over them. THE INTENSITY OF PASSION FOR — Matthew Vantryon, The Indianapolis Star, "Tom Jernstedt, who transformed March Madness and 'loved Indianapolis,' dies at 75," 6 Sep. 2020

Biden has, so far, succeeded where Clinton failed in substituting fervor for the Democratic ticket with fear and loathing of Trump. PASSION — W. James Antle Iii, Washington Examiner, "For Trump, the perks of incumbency might not be enough," 3 Sep. 2020

This general lawlessness gives the sport an idiosyncratic regionalism: College football is barely noticed in some states, and inspires a near-pentecostal fervor in others. PASSION, INTENSITY OF PASSION — Amanda Mull, The Atlantic, "College Football’s Great Unraveling," 14 Aug. 2020

The boat parades have gotten swarms of attention this month, first by possibly setting a world record with more than 1,500 vessels in Florida, and then when boaters swept up in the fervor of a Portland demonstration were saved from drowning. PASSION — Rusty Simmons, SFChronicle.com, "Trump supporters march and float at Golden Gate, call on Democrats to leave party," 30 Aug. 2020

Coach Tom Herman could sense that building fervor in his team Sunday morning. EXCITEMENT, INTENSITY OF PASSION — Nick Moyle, ExpressNews.com, "Longhorns take break from football to focus on social issues," 30 Aug. 2020

And even companies not traditionally involved with fitness are picking up on the at-home workout fervor. PASSION — Rachel King, Fortune, "Most Americans plan to continue at-home workouts even once gyms fully reopen," 17 Aug. 2020

In all MW's examples ARDOR stands for passion, eagerness, zeal, desire, love. FERVOR stands for passion, excitement, the intensity of passion. Importantly, as the ‘examples on the web’ are collected from newspapers, they are biased towards politics and current affairs, and cannot be viewed as a comprehensive illustration. Below, I include examples of literary use from the early 19th century to the early 20th century (I cite Mary Shelley and Herbert Wells, however I have examined also other 19th century and early 20th century writers, including C. Dickens, R. L. Stevenson and P. G. Wodehouse).

The usage cited appears to support the distinction drawn. Both words refer to passion, excitement; ‘ardour’ contains an element of eagerness, desire, love and may refer to the emotion itself; 'fervour' may refer to the level (high) of intensity. Further on the role of etymology, see the P.S.

Shelley, Mary 1797–1851

1. I have read with ardour the accounts of the various voyages which have been made in the prospect of arriving at the North Pacific Ocean through the seas which surround the pole. You may remember, that a history of all the voyages made for purposes of discovery composed the whole of our good uncle Thomas's library.

2. I was easily led by the sympathy which he evinced, to use the language of my heart; to give utterance to the burning ardour of my soul; and to say, with all the fervour that warmed me, how gladly I would sacrifice my fortune, my existence, my every hope, to the furtherance of my enterprise.

3. She was the living spirit of love to soften and attract: I might have become sullen in my study, rough through the ardour of my nature, but that she was there to subdue me to a semblance of her own gentleness.

4. If, instead of this remark, my father had taken the pains to explain to me, that the principles of Agrippa had been entirely exploded, and that a modern system of science had been introduced, which possessed much greater powers than the ancient, because the powers of the latter were chimerical, while those of the former were real and practical; under such circumstances, I should certainly have thrown Agrippa aside, and have contented my imagination, warmed as it was, by returning with greater ardour to my former studies. It is even possible, that the train of my ideas would never have received the fatal impulse that led to my ruin.

5. From this day natural philosophy, and particularly chemistry, in the most comprehensive sense of the term, became nearly my sole occupation. I read with ardour those works, so full of genius and discrimination, which modern enquirers have written on these subjects.

6. As I applied so closely, it may be easily conceived that my progress was rapid. My ardour was indeed the astonishment of the students, and my proficiency that of the masters.

7. These thoughts supported my spirits, while I pursued my undertaking with unremitting ardour. My cheek had grown pale with study, and my person had become emaciated with confinement.

8. I had worked hard for nearly two years, for the sole purpose of infusing life into an inanimate body. For this I had deprived myself of rest and health. I had desired it with an ardour that far exceeded moderation; but now that I had finished, the beauty of the dream vanished, and breathless horror and disgust filled my heart.

9. "These thoughts exhilarated me, and led me to apply with fresh ardour to the acquiring the art of language. My organs were indeed harsh, but supple; and although my voice was very unlike the soft music of their tones, yet I pronounced such words as I understood with tolerable ease.

10. I read of men concerned in public affairs, governing or massacring their species. I felt the greatest ardour for virtue rise within me, and abhorrence for vice, as far as I understood the signification of those terms, relative as they were, as I applied them, to pleasure and pain alone.

11. His soul overflowed with ardent affections, and his friendship was of that devoted and wondrous nature that the worldly-minded teach us to look for only in the imagination. But even human sympathies were not sufficient to satisfy his eager mind. The scenery of external nature, which others regard only with admiration, he loved with ardour:--

12. She had been moved from the posture in which I had first beheld her; and now, as she lay, her head upon her arm, and a handkerchief thrown across her face and neck, I might have supposed her asleep. I rushed towards her, and embraced her with ardour; but the deadly languor and coldness of the limbs told me, that what I now held in my arms had ceased to be the Elizabeth whom I had loved and cherished.

13. He began his lecture by a recapitulation of the history of chemistry, and the various improvements made by different men of learning, pronouncing with fervour the names of the most distinguished discoverers. He then took a cursory view of the present state of the science, and explained many of its elementary terms.

14. My courage and perseverance were invigorated by these scoffing words; I resolved not to fail in my purpose; and, calling on Heaven to support me, I continued with unabated fervour to traverse immense deserts, until the ocean appeared at a distance, and formed the utmost boundary of the horizon.

15. I thought of my father and mother; of you, then my little baby cousin and playmate; and I cried with renewed fervour, till, quite exhausted, I curled myself up under a huge oak among some dry leaves, the relics of a hundred summers, and fell asleep.

16. The Italians were always peculiarly susceptible to these misfortunes. They loved their native walls, the abodes of their ancestors, the familiar scenes of youth, with all the passionate fervour characteristic of that clime.

17. While musing on these things, Idalie’s beseeching eyes were eloquent in imploring him to fly. He consented; but a condition was annexed to his consent, that Idalie should share his flight. He urged his suit with fervour. It were easy for them on a very brief notice to seek the young lady’s confessor, induce him to bestow on them the nuptial benediction, and thus to sanctify their departure together.

18. My father felt this kindness; for a moment ambitious dreams floated before him; and he thought that it would be well to exchange his present pursuits for nobler duties. With sincerity and fervour he gave the required promise: as a pledge of continued favour, he received from his royal master a sum of money to defray pressing debts, and enable him to enter under good auspices his new career.

19. As he spoke, every heart swelled with pride, and every cheek glowed with delight to remember, that each one there was English, and that each supported and contributed to the happy state of things now commemorated. Ryland's fervour increased--his eyes lighted up--his voice assumed the tone of passion.

20. Her eyes glistened with tears when she thanked me, and the grace of her expressions was enhanced, not diminished, by the fervour, which caused her almost to falter as she spoke.

21. I spoke hurriedly, but with fervour: and while with gentle violence I drew her from the portal, some thought, some recollection of past scenes of youth and happiness, made her listen and yield to me; suddenly she broke away with a piercing shriek:--"My child, my child! he has my child; my darling girl is my hostage."

22. Clara, long overcome by excessive grief, had to a great degree cast aside her timid, cold reserve, and received our attentions with grateful tenderness. While Adrian with poetic fervour discoursed of the glorious nations of the dead, of the beauteous earth and the fate of man, she crept near him, drinking in his speech with silent pleasure. We banished from our talk, and as much as possible from our thoughts, the knowledge of our desolation.

23. So we, a simple triad on empty earth, were multiplied to each other, till we became all in all. We stood like trees, whose roots are loosened by the wind, which support one another, leaning and clinging with encreased fervour while the wintry storms howl. Thus we floated down the widening stream of the Po, sleeping when the cicale sang, awake with the stars.

Wells, Herbert George 1866–1946

1. In another moment he had beaten off the Hyena-swine with the handle of his whip, and he and Montgomery were keeping away the excited carnivorous Beast People, and particularly M'ling, from the still quivering body. The hairy-grey Thing came sniffing at the corpse under my arm. The other animals, in their animal ardour, jostled me to get a nearer view.

2. Among other projects that seemed almost equally correct to Kipps at that exalted moment was one of embracing Helen with ardour as soon as the door closed behind her mother and one of headlong flight through the open window. Then he remembered he ought to hold the door open for Mrs. Walshingham, and turned from that duty to find Helen still standing, beautifully inaccessible, behind the tea things.

3. I can remember now my sense of frustration by her unresilient reply. I should have perceived then that for her my ardour had no quickening fire. But how was I to know? I had let myself come to want her, my imagination endowed her with infinite possibilities. I wanted her and wanted her, stupidly and instinctively....

4. You have shared Lady Harman's astonishment at the ardour of his few stolen words in the garden, an astonishment that not only grew but flowered in the silences of her captivity, and you know something of the romantic impulses, more at least than she did, that gave his appearance at the little local railway station so belated and so disreputable a flavour.

5. They found the German tutor in a little court playing Badminton with the two younger boys. He was a plump young man with glasses and compact gestures; the game progressed chiefly by misses and the score was counted in German. He won thoughtfully and chiefly through the ardour of the younger brother, whose enthusiastic returns invariably went out.

6. And so from the prohibition of these acts of folly, on to the prohibition of what I thought then were the maddest, most impossible, and most indecent things one could well imagine. A kind of rhythmic fervour fell on all of us; we gabbled and swayed faster and faster, repeating this amazing Law. Superficially the contagion of these brutes was upon me, but deep down within me the laughter and disgust struggled together.

7. Bechamel laughed awry. Then, with infinite fervour, he said--But let us put in blank cartridge--he said, “------!”

8. Mr. Lewisham was glad she loved reading. He would have been disappointed had she answered differently. But she spoke with real fervour. She loved reading! It was pleasant. She would understand him a little perhaps. "Of course," she went on, "I'm not clever like some people are. And I have to read books as I get hold of them."

9. Mr. Polly backed rather faint-heartedly, but Aunt Larkins was not to be denied. Having hugged and kissed her nephew resoundingly she gripped him by the wrists and scanned his features. She had a round, sentimental, freckled face. "I should 'ave known 'im anywhere," she said with fervour.

10. Suppose he met her when he was out with Helen! "Oh, Lor'!" said Kipps. Life had developed a new complication that would go on and go on. For some time he wished with the utmost fervour that he had not kissed Ann, that he had not gone to New Romney the second time.

11. “Because--they sometimes make a mucker of the cork job, and then the label’s wasted.” “Come and change it, George,” said my uncle, with sudden fervour “Come here and make a machine of it. You can. Make it all slick...”

12. Conviction stormed the last fastness of the disappointed author's mind. "Oh damn!" he shouted with extreme fervour. He had never imagined it was possible that Sir Isaac could come alone.

13. He was full of that old Middle Victorian persuasion that whatever is inconvenient or disagreeable to the English mind could be annihilated by not thinking about it. He used to sit low in his chair and look mulish. “Militarism,” he would declare in a tone of the utmost moral fervour, “is a curse. It's an unmitigated curse.”

P. S.

Some comments on distinctions drawn in other dictionaries and the other answers

MW is the only dictionary that highlights the difference between fervour and ardor in terms of steadiness and continuity, discussed by Sven. The online edition of MW does not appear to cite the basis for the differentiation; the examples provided are insufficient in number, can be read/interpreted differently. The cited MW synonyms dictionary appears to re-state the etymology without saying so expicitly; however, the conclusion, seemingly drawn from the etymology, does not necesarily follow, on the basis of logic, and is not fully supported in the usage. However, the etymology of the two words may suggest difference in quality in the moment, with ardour being flame-like (having a tremulousness of a flame) and fervour - something, simply, extremely heated (possibly boiling-water-like). This is, possibly, why fervor may seem more steady. Importantly, the usage does not seem to suggest that there is necessarily an implication as to being long-term, though it is possible. The usage altogether does not necessarily involve a reference to time. The etymology may also explain the element of eagerness, desire in ardour: the flame is eager to consume more fuel or the object of its desire.

Longman and Collins:

Longman

ar‧dour British English, ardor American English /ˈɑːdə $ ˈɑːrdər/ noun [uncountable] 1 very strong admiration or excitement SYN passion with ardour They sang with real ardour. 2 literary strong feelings of love SYN passion

fer‧vour British English, fervor American English /ˈfɜːvə $ ˈfɜːrvər/ noun [uncountable] very strong belief or feeling religious fervour revolutionary fervour patriotic fervor

Collins

ardour (ɑːʳdəʳ )

REGIONAL NOTE: in AM, use ardor UNCOUNTABLE NOUN Ardour is a strong, intense feeling of love or enthusiasm for someone or something. [literary] ...songs of genuine passion and ardour. ...my ardor for football.

fervour (fɜːʳvəʳ )

REGIONAL NOTE: in AM, use fervor UNCOUNTABLE NOUN Fervour for something is a very strong feeling for or belief in it. [formal] They were concerned only with their own religious fervour.

These two dictionaries touch upon the component of belief in 'fervour', noted in Cambridge; however, in all these instances, in light of the examples provided by the dictionaries, the question of whether it is an intensity of the emotion driven by a belief or the belief itself may need to be asked.

The usage of 'fervour/fervor', historical/literary or MW’s modern, does not indicate that it is necessarily associated with politics, religion, or something similar, and is not always a result of an intellectual conviction or a commitment to a cause (as implied in one of the answers).

P. P. S.

You can examine the usage in plural (ardours, fervours) and the usage by other writers, here Emily Dickinson uses neither word, Dickens and P.G. Wodehouse use both extensively.

Re. whether older literary examples are important: Ray Bradbury's The Smile provides an interesting perspective.