The problem is that gods do operate in different ways than we are used to thinking about them. In our world, gods are typically the monotheistic "fiat lux" type. In DnD, ascension is a valid way of becoming a god. In DnD you get the status of a "god" not for creating the world or making elves, but for granting spells and carrying divine ranks.

Which brings us to the point of whether or not you can believe in gods if Wizards exist with the same powers. The problem is that those Wizards are on the upper percentile of the living, and are likely to become gods anyway. Those "Wizard con men" were probably just that before they became gods.