If you restrict yourself to only songs you identify with, you will have little to sing.

I may well have been the person who made the remark which seems to have been the springboard for the thread.   Or perhaps not.

At any rate, as I--and others--have said before, singing often requires acting.

I'm relatively sure, to say the least,about the imaginary nature of hell but I'm not about to turn down the opportunity to sing the Verdi Requiem, belting out the hellfire and damnation parts with gusto.

And face it, even without roasting in hell, masses are hardcore Catholic theology--and very likely some of the most sublime music ever created.    I--and many other serious musicians, whether professional or just passionate--am completely convinced that religious music is in fact the best vocal music.    You can start with the fact that Tallis and Byrd , both Catholics in a Protestant country, are probably the height of choral music. So if you are unwilling to sing Catholic theology, you don't sing probably the best choral music ever.

So in my group we have Jewish singers, Protestants, agnostics, maybe some atheists, and no doubt some believers, all singing:    "Domine Deus, rex coelestis, Deus pater omnipotens, Domine flli unigenite Jesu Christe....qui tollis peccata mundi" etc. with complete conviction.    Since that is what the music calls for.    We are there to serve the music, not to editorialize.

If you can't put the message across, tell the story convincingly, or convey the emotions of the music, your performance will be lifeless. And the audience will know.

Whether or not you identify with a word of what you are saying.