It starts out with three "alleluia"s, and the melody for the first two of these looked rather familiar to me. They're quite similar to "alleluia"s in Camille Saint-Saëns' Christmas Oratorio.
(notation found here [Palestrina] and here [Saint-Saëns]. The Palestrina notation that I found is in a different time signature than the version in my hymnal, and the setting is slightly different, but the melody itself is the same.)
I've only recently started to really get into sacred music, so I'm not sure if there are particular ways to set particular texts and this just happens to be one of them or if Saint-Saëns wrote his part to resemble Palestrina's. I know virtually nothing about Palestrina and only slightly more about Saint-Saëns.
This - like most of the things I write - is just my stumbling through music trying to understand it better, and I thought this was significant enough to write about.
