I don't think you can attribute most incredible art to be inspired by religion, considering that during those times, art (sometimes just ideas) that lacked religious conformity were dealt with severity. Perhaps the reason there is so much art that is inspired by religion because not much else survived.

This brings a famous bit by George Carlin (I think) to my mind - ...It start's when we are kids, we are told to be creative and to colour within the lines... (paraphrased)