11 “The Lord has made a faithful oath unto David, and He shall not repent”. What means, “has made an oath”? Hath confirmed a promise through Himself. What means, “He shall not repent”? He will not change. For God suffers not the pain of repentance, nor is He deceived in any matter, so that He would wish to correct that wherein He has erred. But as when a man repents of anything, he wishes to change what he has done; thus where you hear that God repents, look for an actual change.
God does it differently from you, although He calls it by the name of repentance; for thou dost it, because you had erred; while He does it, because He avenges, or frees. He changed Saul's kingdom, when He repented, as it is said: and in the very passage where the Scripture says, “It repented Him;” it is said a little after, “for He is not a man that He should repent.” When therefore He changes His works through His immutable counsel, He is said to repent on account of this very change, not of His counsel, but of His work.
But He promised this so as not to change it. Just as this passage also says: “The Lord swore, and will not repent, You are a Priest for ever after the order of Melchizedec;” so also since this was promised so that it should not be changed, because it must needs happen and be permanent; he says, “The Lord has made a faithful oath unto David, and He shall not repent; Of the fruit of your body shall I set upon your seat.” He might have said, “of the fruit of your loins,” wherefore did He choose to say, “Of the fruit of your body”? Had He said that also, it would have been true; but He chose to say with a further meaning, Ex fructu ventris, because Christ was born of a woman without the man.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)