There is no reason why an atheist could not write a good Mass.
I'm really happy with all the art that was inspired by religion, and I think I'd be tickled to little tiny pieces if art in the future was inspired by other things.
As a matter of history great developments in art have often been remarkably separate from religious motivation and use.
Contrary to what many Westerners believe, Islam has a rich tradition of secular painting in spite of its ban on images. It is only in religious rituals that the use of pictorial representation is totally prohibited.
There is no such thing as Christian art or secular art - writing, painting, drawing, whatever it is.
Can a secular artist be used as a vessel to bring souls to Christ? Of course... this is God we are talking about!
I found it amazing people can think that art must be connected to religion. Religion may give art themes, but there would still be art without religion. Bach is not proof that art exists.
Basically there can be no categories such as 'religious' art and 'secular' art, because all true art is incarnational, and therefore 'religious.
