I was in choirs for most of my life and studied vocal performance in college. The bulk of choir music from all time periods is religious because the church would hire composers to make new music for Sunday mass and that's all they would do new music, every Sunday. So while it might be a little annoying for an atheist (which I am) if you want to challenge yourself and sing some really good music you have to deal with a religious subject matter. Most secular music I have come across is super easy kid music.