From: Mark 12:35-37
Christ the Son and Lord of David
 And as Jesus taught in the temple, He said, "How can the scribes say that the Christ is the son of David?  David himself, inspired by the Holy Spirit, declared, `The Lord said to the Lord, Sit at My right hand, till I put Thy enemies under Thy feet'.  David himself calls Him Lord; so how is He his son?" And the throng heard Him gladly.
35-37. Jesus here bears witness, with His special authority, to the fact that Scripture is divinely inspired, when He says that David was inspired by the Holy Spirit when writing Psalm 110. We can see from here that Jews found it difficult to interpret the beginning of the Psalm. Jesus shows the messianic sense of the words "The Lord said to my Lord": the second "Lord" is the Messiah, with whom Jesus implicitly identifies Himself. The mysteriously transcendental character of the Messiah is indicated by the paradox of His being the son, the descendant, of David, and yet David calls Him his Lord. Cf. note on Matthew 22:41-46.
[Note on Matthew 22:41-46 states:
41-46. God promised King David that one of his descendants would reign forever (2 Samuel 7:12ff); this was obviously a reference to the Messiah, and was interpreted as such by all Jewish tradition, which gave the Messiah the title of "Son of David". In Jesus' time this messianic title was understood in a very nationalistic sense: the Jews were expecting an earthly king, a descendant of David, who would free them from Roman rule. In this passage Jesus shows the Pharisees that the Messiah has a higher origin: He is not only "Son of David"; His nature is more exalted than that, for He is the Son of God and transcends the purely earthly level. The reference to Psalm 110:1 which Jesus uses in His argument explains that the Messiah is God: which is why David calls Him Lord--and why He is seated at the right hand of God, His equal in power, majesty and glory (cf. Acts of the Apostles 33-36; 1 Corinthians 6:25).]
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.