Could you please help me analyze this sentence? what does it mean

In the sentence "of choice" and "of the impotent" both describe "the weapon."

Removing "of choice," we have "fervor is the weapon of the impotent."

"Of choice" describes the weapon simply meaning that it is their favorite, best, or most commonly used weapon. See of choice.

The sentence is stating that "fervor" (excitement and passion) is what "impotent" people commonly use.