All art is religious. There. I said it.
Art is a ritual whereby the artist connects to his innermost being, diving deep, mining for gold, or some other precious mineral, eventually to be brought forth to the world.
What is "it" that lies in these inner reaches of man?
The artist seeks to find out. There is no real definition for the stuff that MAKES UP the mind, imagination, and what traditions of old were inclined to call spirit.
But the lack of definition does not degrade the quality of what might be discovered.
re·li·gion - Origin:
Middle English (originally in the sense ‘life under monastic vows’): from Old French, or from Latin religio(n-) ‘obligation, bond, reverence,’ perhaps based on Latin religare ‘to bind.’
The artist must regularly enact the ritual, or practice, to enable the outpouring and refinement of what we call art.
When a something becomes an art, it becomes a practice—a craft. One that must be enacted with constant care. An artist without a "practice" is not an artist at all.
According to Cicero derived from relegere "go through again" (in reading or in thought), from re- "again" + legere "read" (see lecture).
So in this sense, we see that the artist does indeed "go through again". He goes through experiences, perceptions, thoughts, and feelings to make sense of them. Or at least to understand and express those intangibles in some tangible way, however irrational or abstract the end result.
religion (n.) (From Etymology Online):
In that case, the re- would be intensive. Another possible origin is religiens "careful," opposite of negligens.
The artist is careful. He cares for his art and his practice. He nurtures it until he is ready to give birth.