9 “For He is the Lord our God”. But that we may without fear fall down and kneel before Him, what are we? “We are the people of His pasture, and the sheep of His hand.” See how elegantly he has transposed the order of the words, and as it were not given its own attribute to each word; that we may understand these very same to be the sheep, who are also the people. He said not, the sheep of His pasture, and the people of His hand; which might be thought more congruous, since the sheep belong to the pasture; but He said, “the people of His pasture.”
The people are therefore sheep, since he says, “the people of His pasture:” the people themselves are sheep....He praises these sheep also in the Song of Solomon, speaking of some perfect ones as the teeth of His Spouse the Holy Church: “Your teeth are like a flock of sheep that are even shorn, which come up from the washing; whereof every one bears twins, and there is none barren.” What means, “Your teeth”? These by whom you speak, for the teeth of the Church are those through whom she speaks.
Of what sort are your teeth? “Like a flock of sheep that are shorn.” Why, “that are shorn”? Because they have laid aside the burdens of the world. Were not those sheep, of which I was a little before speaking, shorn, whom the bidding of God had shorn, when He says, “Go and sell that you have, and give to the poor; and you shall find treasure in heaven: and come and follow Me”? They performed this bidding: shorn they came. And because those who believe in Christ are baptized, what is there said? “which come up from the washing;” that is, come up from the cleansing. “Whereof every one bears twins.” What twins? Those two commandments, wherefrom hang all the Law and the Prophets.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)