2 But when the Jews could not answer the Lord proposing a question, and asking “whose Son they said Christ was;” and they answered, “the Son of David;” He goes on with the further question put to them, “How then does David in spirit call Him Lord, saying, The Lord said unto my Lord, Sit on My right hand till I make Your enemies My footstool. If David then,” He says, “in spirit call Him Lord, how is He his Son?” He did not say, “He is not his Son, but how is He his son?”
When he says “How,” it is a word not of negation, but of enquiry; as though He should say to them, “You say well indeed that Christ is David's Son, but David himself does call Him Lord; whom he then calls Lord, how is He his Son?” Had the Jews been instructed in the Christian faith, which we hold; had they not closed their hearts against the Gospel, had they wished to have spiritual life in them, they would, as instructed in the faith of the Church, have made answer to this question and said, “Because in the beginning was the Word, and the Word was with God and the Word was God:” see how He is David's Lord.
But because “The Word was made flesh, and dwelt among us;” see how He is David's Son. But as being ignorant, they were silent, nor when they shut their mouths did they open their ears, that what they could not answer when questioned, they might after instruction know.
Source: Sermons on Selected Lessons of the New Testament (New Advent)