What is the phrase for 'essential spark' or 'unifying vision' that unique pieces of writing have that bring people together around a cause?

The “unifying vision” that brings people together around a cause can be called a rallying cry:

A rallying cry or rallying call is something such as a word or phrase, an event, or a belief which encourages people to unite and to act in support of a particular group or idea.
_{Source: Collins—rallying cry}

Examples:

In the iconic speech Dr. King publicly declared, “We shall overcome” – a phrase that became a rallying cry for the civil rights movement.
_{Source: The University of Memphis—”Original Handwritten Martin Luther King Jr. Speech to be Displayed . . .”}

Recorded in a Montreal hotel room during his and Yoko Ono’s honeymoon, John Lennon’s “Give Peace a Chance” was a rallying cry—or as he described it, a commercial for peace.
_{Source: Apple Music—”Give Peace a Chance”}

If you’re looking for “essential qualities” shared by these writings, try:

inspirational—giving you the enthusiasm to do or create something
_{Source: MacMillan—inspirational}

motivational—designed to promote the desire or willingness to do or achieve something.
_{Source: Lexico—motivational}

Examples:

John Lennon’s inspirational song . . .

Martin Luther King Jr.’s motivational speech . . .

Antonyms for inspirational and motivational include dull and unaffecting (like your 90s corporate mission statement, perhaps).

.

PS: I might label Adam Smith’s concepts (along with those of Calvin and Hobbes) as visionary or influential or ideological rather than inspiring:

? Smith’s “invisible hand” concept makes us want to go shopping!