According to each of the Synoptic writers, Jesus posed two questions to the Pharisees about the Christ which they found unanswerable: “How then does David in the Spirit call Him ‘Lord,’ saying: ‘The Lord said to my Lord, “Sit at My right hand till I make Your enemies Your footstool”’? If David then calls Him ‘Lord,’ how is He his Son?” (Matthew 22:43-45). As Peter points out in Acts 2, this promise was not made to David; he would not ascend into the heavens and sit at God’s right hand (v. 34). Obviously, therefore, it was made to one of David’s descendants, the Christ.
The point of Psalm 110:1 is that of the dual nature of the Christ; He would be both human (David’s physical descendant) and deity (a member of the Godhead, a peer of Jehovah). Or as it more often stated in the New Testament: Jesus is the Son of Man (i.e. human) and the Son of God (i.e. deity).
The gospel which the Lord God promised before through His prophets in the Holy Scriptures was focused upon “His Son Jesus Christ our Lord, who was born of the seed of David according to the flesh, and declared to be the Son of God with power according to the Spirit of holiness, by the resurrection from the dead” (Romans 1:3-4).
Therefore, know assuredly that this man Jesus of Nazareth is both Lord and Christ.