Based on programs I've found on the web, I'm not the first to immediately think of Palestrina's motet O Rex gloriae (and his Mass setting based on it) as the Feast of Christ the King approaches. Of course, the text of the motet is from the Vespers antiphon for the Ascension:

"O Rex gloriae, Domine virtutum,
qui triumphator hodie super omnes caelos ascendisti,
ne derelinquas nos orphanos:
sed mitte promissum Patris in nos,
Spiritum veritatis."

"O King of glory, Lord of Hosts,
you who this day ascended triumphantly above all the heavens,
leave us not orphans,
but send upon us the Promise of the Father,
the Spirit of truth."

At my prompting, my Latinist friend, Marion Smedberg, suggested this slight alteration to make the motet more appropriate for Christ the King:

"qui triumphator hodie super omnes sanctos coronatus"

"you who today as triumpher (have been) crowned above all the saints"

The rest, though clearly anticipating Pentecost, seems perennial enough a sentiment to use anytime. Any of you sticklers for Latin syntax (and/or theology, for that matter) who would like to nay say, have at it.