Mark 12:36
For David himself said by the Holy Ghost, The LORD said to my Lord, Sit thou on my right hand, till I make thine enemies thy footstool. For David himself said by the Holy Ghost, The LORD said to my Lord, Sit thou on my right hand, till I make thine enemies thy footstool.
David spoke the words by the Holy Spirit, by the Spirit of inspiration from God. When questioned whose son Christ is, the Pharisees identified him as the son of David, the title prophecy gave to the Messiah. Jesus used the words to test and challenge whether the Messiah is only David's human son and merely a man or is the divine Son of God.
By calling the Messiah his Lord, David shows the Messiah is David's Lord and therefore more than David's son, more than a human descendant, and more than a mere man, even the divine Son of God. The saying enthrones him at God's right hand until his enemies are subdued and made his footstool. Jesus pressed the argument with the question how he is his son if David calls him Lord, and they were unable to answer a word.