The “Great Catholic Mass” of JS Bach is clearly not intended for the Catholic rite, especially as the words of the Sanctus clearly follow the Lutheran rather than the Roman formula. The facts of the matter seem to be that the finished work was compiled in the 1740s drawing on a huge number of previous borrowings (many of which seem to be lost):
I. In 1733, Bach had approached the Elector of Dresden for a title in the court chapel with a setting of the Lutheran Missa, namely Kyrie and Gloria, in order to gain traction in his ongoing battles with the Leipzig authorities. (Note that Bach’s other four “mass” settings BWV 233-236 are just the movements of the Lutheran Missa, i.e. Kyrie and Gloria only.)
II. The Symbolum Nicenum seems to have been compiled from a variety of pre-existing movements, some of which are related to the Creed (e.g. as the opening section quotes one of the Gregorian intonations). Only the Confiteor appears to be newly composed.
III. The Sanctus (without Osanna, and with the words “gloria ejus” instead of “gloria tua” immediately preceding where the “Osanna” would follow on) had already been composed for Christmas 1724, and is the best of his several settings of the text.
IV. This left JSB with four sections to fill in, again with pre-existing choruses or arias (and the re-use of the Gratias as Dona nobis pacem).
In this respect the Mass resembles several other of Bach’s late projects such as the Art of Fugue, or the Musical Offering, which Bach seems to have compiled following his own artistic needs, rather than having to consider the practicality of performance.
Interestingly, most of Mozart’s settings are prior to the reign of Emperor Joseph II, who officially criticised elaborate music in church services; this is partly why there is a significant gap in church composition in the œuvres of both Joseph Haydn and Mozart. Mozart, of course, did not outlive Joseph significantly, so that only a few settings of the Kyrie (most incomplete) date from the 1780s, and neither the 1783 Mass nor the 1791 Requiem were completed; Haydn wrote his final six Esterhazy masses in the mid 1790s to early 1800s, and wonderful works they are, though they are also rather long for ecclesiastical use, mostly being about three-quarters of an hour in duration. Mozart’s 1783 setting of the “great” mass was actually planned as a nuptial thanksgiving after his marriage to Constanze née Weber, though the circumstances of its first performance in Salzburg seem to indicate that the usual restriction on the length of the musical offering was relaxed for the prodigal son to perform it .
Archbishop Colloredo of Salzburg was known to have discouraged composers from writing masses with more than 20 to 25 minutes duration, because he didn't want the entire liturgy to occupy longer than an hour and a half, so most (but not all) of Mozart’s Salzburg masses are of the brevis variety; despite this, he never adopted the brothers Haydn’s rather naughty trick of telescoping the long texts of the Gloria and Credo, where each section of the choir sings a different portion of the text...