If the words come from Christian worship but aren't otherwise objectionable, can you use them for secular music?
Asking this question reminded me of another.
As part of my (past) musical studies, I've studied historical compositions that set the "ordinary" of the Christian mass to music. The "ordinary" consists of five key prayer texts that are frequently (always?) part of the mass. Four of them are blatantly problematic for Jews in terms of content. The fifth, Sanctus (Wikipedia), focuses on God's glory and doesn't talk about other parts of the Christian trinity. (It's clearly related to our Kedusha.) As a student studying this music for its musical, not textual, properties, I once composed a setting of the Sanctus text. (Sure wasn't going to choose one of the others! That was a visceral reaction; I was not observant then.)
Looking back on that time now, I wonder whether setting a text that contains no objectionable religious references, but comes from a religious context, is permitted. Does context matter -- university assignment versus personal project, where (or if) it will be performed, perhaps other factors?
(Just in case anybody is concerned, this is not a practical matter for me. I am no longer a university student and when I want to do choral composition I use more suitable texts. I'm just curious.)