Eloquence was born before the rules of Rhetoric, as languages were born before Grammar.
Nature makes man eloquent in matters of great interest or strong passion. Whoever is greatly excited sees things with a different eye from everyone else. For him, everything is seen comparatively or metaphorically: without noticing, he becomes animated and shares a part of his enthusiasm with others.
Voltaire’s strong passions and great interests certainly explain his eloquence, and probably his long and productive life, as well.