We now return to the Trinity Preface during all these Sundays post Trinitatem: we shall know it quite well by December! Whence cometh it?
There are phrases from the Trinity Sunday preface within the Stowe Missal. This is a liturgical book scribed in the 790s, but copied undoubtedly from a much earlier source (for example, it lacks evidence of the Gregorian changes, such as the moving of the Pater Noster to a place immediately after the Roman Canon and before the Fraction). It may have been the book of an itinerant priest in South-West Ireland, or one who could not afford a large Mass-book (it only has one Epistle and Gospel, but is clearly meant for all-the-year use since it has the seasonal paragraphs of the Communicantes). And the Preface in this rite goes as follows:
" ... through Christ our Lord: who with thine only begotten Son and the Holy Ghost art one God and immortal God and incorruptible and immovable God and invisible and faithful God [much more like this up to:] good and holy God, not singulariter of one person but of the one substance of the Trinity; Thee we believe, Thee we bless; Thee we adore [etc.]."
This faintly reminds me of the same place in the Anaphora of S John Chrysostom:
" ...For thou art God ineffable, incomprehensible,invisible, inconceivable, ever being as Thou art, Thou and thine only-begotten Son and thy Holy Spirit [etc.]."
Is this simply a generic similarity, of Christians toto orbe divisi thinking the same way in the same context, or is there a textual link? I strongly incline to the former possibility.
The preface of the Trinity, in more or less its present form, is in the Gelasian Sacramentary, for use on the Sunday after Pentecost. Is it from a source like this that Alcuin borrowed it for the Sunday Mass of the little book which I am sure (pace G G Willis) he put together for priests minimally equipped during the Carolingian period?
A brief look at the manuscript evidence suggests to me that the rubric directing this Preface to be used on the Sundays after Trinity (aka after the Octave of Pentecost) first appears in Sarum Missals early in the 14th century ... as one might expect. It is also in the Westminster and Hereford Missals; was it universal in the later English Middle Ages?
We then have a gap until Pope Clement XIII made it the Green Sunday (and Advent etc.) Preface in the Missale Romanum in 1759. Had there been continuity anywhere in this custom between the Tridentine curtailing of local rites, and the 18th century ... or did it just occur to the pope quite spontaneously that it was a good idea to make the Trinity Preface the all-purpose Sunday Preface? After all, Sunday commemorates the Creation, the Resurrection, the advent of the Spirit ...