26 We may perceive also, from the words of our Lord Himself, why He said, You are not far from the Kingdom of Heaven, rather than, You shall be in the Kingdom of Heaven. Then follows: And no man after that dared ask Him any question. And Jesus answered and said, as He taught in the Temple, How say the Scribes that the Christ is the Son of David? David himself says in the Holy Spirit, The Lord said unto my Lord, Sit on My right hand, till I make Your enemies the footstool of Your feet . David himself calls Him Lord, and whence is He his Son ? The Scribe is not far from the Kingdom of God when he confesses one God, Who is to be loved above all things.
But his own statement of the law is a reproach to him that the mystery of the law has escaped him, that he does not know Christ the Lord, the Son of God, by the nature of His birth to be included in the confession of the one God. The confession of one God according to the law seemed to leave no room for the Son of God in the mystery of the one Lord; so He asks the Scribe, how he can call Christ the Son of David, when David calls Him his Lord, since it is against the order of nature that the son of so great a Patriarch should be also his Lord.
He would bid the Scribe, who regards Him only in respect of His flesh, and His birth from Mary, the daughter of David, to remember that, in respect of His Spirit, He is David's Lord rather than his son; that the words, Hear, O Israel, the Lord our God is one Lord, do not sever Christ from the mystery of the One Lord, since so great a Patriarch and Prophet calls Him his Lord, as the Son begotten of the Lord before the morning star. He does not pass over the law, or forget that none other is to be confessed Lord, but without violating the faith of the law, He teaches that He is Lord, in that He had His being by the mystery of a natural birth from the substance of the incorporeal God. He is one, born of one, and the nature of the one Lord has made Him by nature Lord.
Source: On the Trinity (New Advent)