John Chrysostom

And that they may not say, that it was in flattery he so called Him, and that this was a human judgment, see what He saith, "How then doth David in spirit call Him Lord?" See how submissively He introduces the sentence and judgment concerning Himself. First, He had said, "What think ye? Whose Son is He?" so by a question to bring them to an answer. …

Homily on the Gospel of Matthew 71 ↗

Jerome

This passage is out of the 109th Psalm. Christ is therefore called David's Lord, not in respect of His descent from him, but in respect of His eternal generation from the Father, wherein He was before His fleshly Father. And he calls Him Lord, not by a mere chance, nor of his own thought, but by the Holy Spirit.

Catena Aurea by Aquinas ↗

Thomas Aquinas

Then he objects in order to give them to understand the other generation: how then doth David in spirit call him Lord, saying: the Lord said to my Lord: sit on my right hand? Psalm 109:1. It is found in the law that the father is greater than the son. Therefore the son is not lord of the father. Therefore either Christ is not the son of David, or there is something in him greater than David, since he calls him Lord. …

Commentary on Matthew ↗