Idk based on some of the selected language you used I wouldn't be surprised if it was the edgy atheist table. "Taken back" at "inserting faith" like any player would know what that meant in a DMs world? Unless you require every player to choose the select religious from your pre generated pantheon/word I cant sympathize with you. This wouldn't be much different if he chose any god from any system that you would be unfamiliar with. What if he chose Lathander? Or Zeus? Is buddhism in your game? Literally every complaint you could possibly have would be similar if he chosen one of those gods. I've never seen this sub support forced pantheons.

I've seen it too often but hey it's your game. The only real problem is when people take it literally. I'm assuming if he is actually wanting to be a rabi there are issues he will have to reconcile just like all characters that have any backbone or defining trait. Every character with a defining trait will have to role play those situations so I can't see why you would just assume by default he would fail his roleplay. What is the difference in your mind between yahweh and Zeus? I dont find any of your answers satisfactory. Should I just assume any character who worships thor will just start attacking everything 0 role play? Why are you assuming he will just Rabi everything to death (or whatever that means)?

I just cant really see this being an issue especially with daily threads about "wow dm believes I need to have an oath or religion to shoot sunbeams I'm so angry >:c." Paladin and cleric threads always end on some angry derailed tangent because of these discussions. People have even concluded that the "divine power" or "patron power" is just "from within" or whatever that means. So if it can just spontaneously come from nothing I see no reason his character could simply believe God or Yahweh existed and your world rationalizes it however it wants.

Tldr: What? I cant see this being a problem unless you actually made a pantheon of gods and require every player to adhere to it. Or if the player overzealously plays a 1 dimensional character. There is no difference than this being the chosen god "zeus" or "thor"