37 “And He took him from the flocks of sheep, from behind the teeming sheep He received him: to feed Jacob His servant, and Israel His inheritance”. This David indeed, of whose seed the flesh of Christ is, from the pastoral care of cattle was translated to the kingdom of men: but our David, Jesus Himself, from men to men, from Jews to Gentiles, was yet according to the parable from sheep to sheep taken away and translated. For there are not now in that land “Churches of Judæa in Christ,” which belonged to them of the circumcision after the recent Passion and Resurrection of our Lord, of whom says the Apostle, “But I was unknown by face to the Churches of Judæa, which are in Christ,” etc. Already from hence those Churches of the circumcised people have passed away: and thus in Judæa, which now does exist on the earth, there is not now Christ. He has been removed thence, now He does feed flocks of Gentiles.
Truly from behind teeming sheep He has been taken thence. For those former Churches were of such sort, as that of them it is said in the Song of Songs, “Your teeth— are like a flock of shorn ewes going up from the washing, all of which do bear twins, and a barren one is not among them.” For they then laid aside like as it were fleeces the burdens of the world, when before the feet of the Apostles they laid the prices of their sold goods, going up from that Laver, concerning which the apostle Peter does admonish them, when they were troubled because they had shed the blood of Christ, and he says, “Repent ye, and let each one of you be baptized in the name of the Lord Jesus Christ, and your sins shall be forgiven you.” But twins they begot, the works, to wit, of the two commandments of twin love, love of God, and love of one's neighbour: whence a barren one there was not among them.
From behind these teeming sheep our David having been taken, does now feed other flocks among the Gentiles, and those too “Jacob” and “Israel.” For thus has been said, “to feed Jacob His servant, and Israel His inheritance.”...Unless perchance any one be willing to make such a distinction as this; viz. that in this time Jacob serves; but he will be the eternal inheritance of God, at that time when he shall see God face to face, whence he has received the name Israel.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)