Matthew 22:45

Then since He had answered, He asks also in turn, "What think ye of Christ, whose Son is He? They say unto Him, The Son of David." See after how many miracles, after how many signs, after how many questions, after how great a display of His unanimity with the Father, as well in words, as in deeds; after having praised this man that said, that there is one God, He asks the question, that they may not be able to say, that He did miracles indeed, yet was an adversary to the law, and a foe to God. Therefore, after so many things, He asks these questions, secretly leading them on to confess Him also to be God. And the disciples He asked first what the others say, and then themselves; but these not so; for surely they would have said a deceiver, and a wicked one, as speaking all things without fear. So for this cause He inquires for the opinion of these men themselves. For since He was now about to go on to His passion, He sets forth the prophecy that plainly proclaims Him to be Lord; and not as having come to do this without occasion, nor as having made this His aim, but from a reasonable cause. For having asked them first, since they answered not the truth concerning Him (for they said He was a mere man), to overthrow their mistaken opinion, He thus introduces David proclaiming His Godhead. […]

"If David then call Him Lord, how is He then his Son," not taking away the fact that He is his Son, away with the thought; for He would not then have reproved Peter for this, but to correct their secret thoughts. So that when He saith, "How is He his Son?" He meaneth this, not so as ye say. For they said, that He is Son only, and not also Lord. And this after the testimony, and then submissively, "If David then call Him Lord, how is He his Son?" But, nevertheless, even when they had heard these things, they answered nothing, for neither did they wish to learn any of the things that were needful. Wherefore He Himself addeth and saith, that "He is his Lord." Or rather not even this very thing doth He say without support, but having taken the prophet with Him, because of His being exceedingly distrusted by them, and evil reported of amongst them.

(verses 41 onwards) But when the Pharisees were gathered together, Jesus questioned them, saying, 'What do you think about the Christ, whose son is he?' They said to him, 'David.' He said to them, 'How then does David in the Spirit call him Lord, saying, "The Lord said to my Lord, Sit at my right hand, until I make your enemies your footstool?" If David then calls him Lord, how is he his son?' Those who had gathered together to test Jesus and tried to capture the truth through deceitful questioning, gave an opportunity for their own refutation.

Here then there is need for caution, lest Christ himself be thought to have denied that he was the Son of David. He did not deny that he was the Son of David, but he probed his detractors on the particular way this can be. You have said that Christ is the Son of David. I do not deny it. But "if David thus calls him Lord, how is he his Son?" Tell me how he could be his son who is also his Lord? They did not answer him but were dumbfounded.Let us then answer them by the explanation given by Christ himself. Where given? Through his apostle. By what source can we prove that Christ himself has explained it? The apostle says, "Would you receive a proof of Christ who speaks in me?" So it is through the apostle's voice that Christ has allowed this question to be solved. In the first place, do you remember what Christ said, speaking by the apostle to Timothy? "Remember Jesus Christ, raised from the dead, descended from David. This is my gospel." So it is easy to see that Christ is the Son of David. But how is he also David's Lord? Let the apostle again tell us of the one who, "though he was in the form of God, did not count equality with God a thing to be grasped." Acknowledge David's Lord. If you acknowledge David's Lord, our Lord, the Lord of heaven and earth, the Lord of the angels, equal with God, in the form of God, how is he David's Son? Note what follows. The apostle shows you David's Lord by saying, "Who being in the form of God, thought it not robbery to be equal with God." And how is he David's Son? "But he emptied himself, taking the form of a servant, being made in the likeness of men; and being found in human form, he humbled himself, having become obedient unto death, even the death of the cross. Therefore God has also highly exalted him." Christ "of the seed of David," the Son of David, rose again because "he emptied himself." How did he empty himself? […]

(ap. Anselm.) He concludes from this authority, If David then call Him Lord, how is He his son?