In Years A and B for the third Sunday in Ordinary Time, the Introit is Dominus secus mare Galilaéae rather than Adorate Deum, the option for Year C and The Third Sunday after Epiphany. This Introit is highly unusual in that it employs a Gospel text rather than a Psalm. It is even more uncommon that this text from Matthew’s Gospel sung at the beginning of Mass will then be heard again when recited by the celebrant at the Gospel (this time from Mark), and for a third time (naturally) at Communion.
“The Lord walking by the sea of Galilee, saw two brothers, Peter and Andrew, and he called out to them: “Follow me, and I will make you fishers of men.”
This unique Introit cannot be found anywhere in the Liber Usualis. In the Graduale Triplex, it is documented from the Einsiedeln manuscript which was begun in the 11th Century and is believed to the oldest most complete Missal of Gregorian Chant in existence. So where has it come from?
UPDATE – Good old Stan Metheny has answered my question. It comes from the Mass formulary of the Vigil of St Andrew the Apostle (29th November) in the older (1961) graduale. (I don’t have one of those…) Stan writes:
The melody moves along at a steady narrative pace, through the ornamentation over duos fratres and vos… piscatores, evoke images of fisherman raising and lowering their nets out of the water.
See the rest of Stan’s analysis on the Communion Antiphon in the comments box.