35 Lastly there follows, “and He built like as of unicorns His sanctification”: or, as some interpreters have made thereof a new word, “His sanctifying.” The unicorns are rightly understood to be those, whose firm hope is uplifted unto that one thing, concerning which another Psalm says, “One thing I have sought of the Lord, this I will require.” But the sanctifying of God, according to the Apostle Peter, is understood to be a holy people and a royal priesthood. But that which follows, “in the land which He founded for everlasting:” which the Greek copies have εἰς τὸν αἰῶνα, whether it be called by us “for everlasting,” or “for an age,” is at the pleasure of the Latin translators; forasmuch as it does signify either: and therefore the latter is found in some Latin copies, the former in others. Some also have it in the plural, that is, “for ages:” which in the Greek copies which we have had we have not found. But which of the faithful would doubt, that the Church, even though, some going, others coming, she does pass out of this life in mortal manner, is yet founded for everlasting?
36. “And He chose David His servant”. The tribe, I say, of Judah, for the sake of David: but David for the sake of Christ: the tribe then of Judah for the sake of Christ. At whose passing by blind men cried out, “Have pity on us, Son of David:” and immediately by His pity they received light, because true was the thing which they cried out. This then the Apostle does not cursorily speak of, but does heedfully notice, writing to Timothy, “Be thou mindful, that Christ Jesus has risen from the dead, of the seed of David,” etc. Therefore the Saviour Himself, made according to the flesh of the seed of David, is figured in this passage under the name of David, the Lord opening in parables His mouth. And let it not move us, that when he had said, “and He chose David,” under which name he signified Christ, he has added, “His servant,” not His Son. Yea even hence we may perceive, that not the substance of the Only-Begotten coeternal with the Father, but the “form of a servant” was taken of the seed of David.
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.
38. “And He fed them,” he says, “in the innocence of His heart”. What can be more innocent than He, who not only had not any sin whereby to be conquered, but even not any to conquer? “And in the understanding of His hands He led them home:” or, as some copies have it, “in the understandings of His hands.” Any other man might suppose that it would have been better had it been said thus, “in innocence of hands and understanding of heart;” but He who knew better than others what He spoke, preferred to join with the heart innocence, and with the hands understanding. It is for this reason, as far as I judge; because many men think themselves innocent, who do not evil things because they fear lest they should suffer if they shall have done them; but they have the will to do them, if they could with impunity. Such men may seem to have innocence of hands, but yet not that of heart. And what, I pray, or of what sort is that innocence, if of heart it is not, where man was made after the image of God? But in this which he says, “in understanding (or intelligence) of His hands He led them home,” he seems to me to have spoken of that intelligence which He does Himself make in believers: and so “of His hands:” for making does belong to the hands, but in the sense wherein the hands of God may be understood; for even Christ was a Man in such sort, that He was also God....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)