I think I can make a recommendation here that actually meets the terms of the original request, way, way upstream, both for an artist and a specific song or two.

The artist is Cat Faber, who's known in the SF/fantasy "filk" community as a first-rate musician and song-crafter, both on her own and previously as part of the duo Echo's Children. One long-running thread in her work is the importance of human action as opposed to dependence on the divine, and there is among her newer works in fact an Atheist's Anthem. A look through her other work, as archived on the linked Web site, may also prove fruitful. I would also suggest looking up the Echo's Children album A Dancing World; there are several songs on that album that may be of interest.