Ps 77:70

“Lastly there follows, "and He built like as of unicorns His sanctification" [Psalm 78:70]: 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. [1 Peter 2:9] 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?”

“David sheweth in a psalm that he was chosen from a station which taught simplicity, for he was chosen from following the sheep, even as he himself confesseth and calleth to mind his election in one of his psalms, saying, "He chose David His servant, and took him from following a flock of sheep, and from after the ewes that gave suck."”