Well, I'd say theism and religion aren't the sources of artistic inspiration, but hijackers of the true sources. Religious experience is due to peak states of consciousness which are biological and can be triggered by recreational drugs. That's how religion got started, through shamanism. Likewise, the concept of God is a bastardization of the natural sense of the sublime, of the strangeness of anything's being (including God's).
Artists draw from religious stories and traditions to get paid under theocracy, but the sources of artistic inspiration are existential, psychological, and natural (as well as political, as in the case of social criticism in art).