jbaldy22- thanks for the acknowledgment.It is an important distinction because a number of jewish singers today (many chareidim) have used “outside” tunes and set them to hebrew verses. Obviously, they will not be barred from the “Amud’ !
Itchesrulik: As far as I know, the only “piyut’ that may be sung as per an old non-Jewish tune is “Maoz Tsur” on chanukkah.
I have no knoweldge of “church music’ being used elsewhere.