Changing the definition of what a deity is is moving the goal posts. That doesn't fly.
First, we have to recognize that deities factually exist in most D&D cosmologies. Every book calls them deities. Every stat block lists them as deities. The DM and players, external to the imaginary universe of the game, know that these entities being deities is as factual as the Dex bonus an Elf gets, or the d8 damage a sword deals. It's not a matter for us as players to debate whether or not they *are* deities. By definition they are. They do gain power when worshiped. They have dominion, sometimes absolute dominion, over some realm of existence. They are literally de jure deities because the rulebook says they are. However you choose to define "deity" you have to include these entities in that definition or you've fundamentally altered the cosmology of the campaign setting.
The question, then, is, "How can a character who does not know the truth establish the truth?"