Long ago mankind created beings believed to be perfect, and are now worshiped as gods. But when a man discovers their secret, he begins to question their divinity.

Good note FFF. Fights make it less passive more active.

I like your rewriting! I wouldn't say "tries to expose", I would say "fights" or "struggles".

"When he discovers the gods on Olympus are not divine, an ex-soldier turned priest vows to take then down; to free mankind from their fraudulent rule."