In setting the Mass Ordinary, what are the words that should be set in a fugue? "Cum Sancto Spiritu in gloria Dei Patris" or just "in gloria Dei Patris"? That's to say, is the entire Cum Sancto Spiritu set fugally, or just when the words "in gloria Dei Patris" arise?
I can sympathise with this question, because it is frequently difficult to determine what is required in liturgical music, and what is actually a tradition that developed by accretion. In a field as conservative as religious music can be, at times tradition can even take on the force of law...
In this particular case, and bear in mind that I am not a scholar of liturgical music, I would have to venture that only requirement for the Cum Sancto Spiritu section is that it be choral rather than solo. The Germans from about Bach on seemed to set the words "Cum Sancto Spiritu" homophonically, and reserve fugal textures for "in gloria Dei Patris". I've noticed that both Bach's B minor Mass and Haydn's C Major Mass feature this sort of structure. I wouldn't be surprised if others did the same later, given Bach's prestige.
Vivaldi, in the Gloria RV 589, however, writes fugally throughout, and, if you go back to the start of imitative writing in Masses, Josquin's Cum Sancto Spiritu from the Missa de Beata Virgine uses fugal texture from the beginning as well, and this particular piece was very much admired, "covered" and imitated - there is a nice keyboard tiento on it by Cabezón.
So, I think you have some freedom in how to tackle this if you are writing a Mass of your own.