Sure, I'll agree with you that a religion check would be appropriate. Which is my main point: the player should have some way of being informed about Shar. If Tav should know about Shar, then the player definitely should.
And even if (your) Tav shouldn't know about Shar, the player should still be given information about Shar through journals/pop-up text/dialogue, so they can have the choice of how to roleplay Tav.
