I've heard/sung many different versions of "Requiem" (Mozart, Brahms, Rutter, Faure). They are usually sung in latin with the same lyrics, except "Kyrie Eleison" which is greek.
So why is a greek phrase always included in an otherwise all-latin piece of music?
Wikipedia gives this for Kyrie...
In Rome, the sacred Liturgy was first celebrated in Greek. As Christianity gained popularity, the Roman Mass was translated into Latin, but the familiar and venerated Greek prayer Kýrie, eléison was preserved,[citation needed] as were Hebrew phrases such as "Alleluia" and "Hosanna".
Though it says 'citation needed', it might be a hint in the right direction.
Disclaimer: My knowledge of Christianity is not comprehensive, nor am I a subscriber. I tend to listen to a Requiem as a 'song' rather than a 'prayer'
I must add this shameless plug for a piece written by a friend of mine - Requiem & Kyrie, Michael Yates [free to listen, & on a well-respected Library music site, de Wolfe] which was an 'overture' in effect for a complete Requiem suite
The phrase "Kyrie Eleison" comes from Greek: Κύριε ἐλεήσον. The first word, Κύριε, is the vocative form of "Lord", which means God is being addressed. The second word, ἐλεήσον, is an imperative which means "have mercy". Put it together, and you get "Lord, have mercy".
This is a common prayer in Christian liturgy, so it makes sense that it's a lyric in requiem masses. As for why the Greek words were not translated into Latin, to be like the rest of the lyrics, the answer seems to be found on Wikipedia.
In Rome, the sacred Liturgy was first celebrated in Greek. As Christianity gained popularity, the Roman Mass was translated into Latin, but the familiar and venerated Greek prayer Kýrie, eléison was preserved,[citation needed] as were Hebrew phrases such as "Alleluia" and "Hosanna". (Wikipedia)
In a nutshell, the lyrics of requiem masses are borrowed from Christian liturgy. The Roman Catholic Church saw fit to preserve this Greek phrase in their liturgy, even when the rest of it was translated into Latin. It's a famous phrase and, in my opinion, one worth knowing!
