I do the same about Knowledge checks.
But I don't actually see the issue you mention. It is not at all a problem that a Wizard (or a Rogue) knows a little bit more than a Cleric, when they are all trained in Religion. Why shouldn't the char with higher Int learn faster than the char with lower Int? There is nothing that makes a Cleric inherently more entitled at Knowledge (Religion). The Cleric could be inherently entitled on knowledge about her own religion, but not the religions of others.
Well...here is where it gets dicey, imho: You are basically forcing clerics to be focused on Wisdom, because their core mechanic, spellcasting, uses Wisdom. You are immediately putting any cleric that wishes to be on par with the other characters from a mechanical sense into a Wisdom bottleneck. You are saying, from the get-go, that all clerics are going to be more insightful, more perceptive, better with animals, better at medicine, and better at survival. Across the board. Just like all wizards will be better than clerics at religious knowledge if they are equally trained, simply because there is a severely strong incentive to not pump your Intelligence as a cleric.
The problem is that you are, right out of the gate, eliminating a ton of very common types of characters throughout fiction, and throughout history for that matter. Because clerics, to function properly within the mechanical framework of the game, really have to invest in being wise. Else, they'll be hamstringing themselves, like a fighter trying to work with a 10 Strength (assume a non-finesse fighter). Can such a fighter do stuff in the world, including succeed in combat? Sure. But they are going to be dramatically worse at it than a fighter who has invested in Strength.
And actually, I think you are wrong about how someone versed in religion, even mostly their own, would not be better at understanding other religions inherently.
It's like language. Linguists get a few disparate languages under their belt, a core language from a region of the world, and they hit a flashpoint of being able to swiftly learn new languages with commonalities with all the languages they do understand. By virtue of understanding the foundation of how language tends to work to begin with.
Similarly, a person well-versed in their own religion, or more likely in a handful of disparate religions, will have a much better idea what to expect from any new religion they come across, than the most intelligent of individuals who are not particularly well-versed in religious studies.