While Jesus was teaching in the temple courts, he asked, “Why do the teachers of the law say that the Messiah is the son of David? David himself, speaking by the Holy Spirit, declared:

“‘The Lord said to my Lord:
    “Sit at my right hand
until I put your enemies
    under your feet.”’

David himself calls him ‘Lord.’ How then can he be his son?”

The large crowd listened to him with delight. Mark 12:35-40

            Jesus quoted Psalm 110:1 when asked a question by the Pharisees—how could the Messiah be the son of David.  Jesus questioned them by asking how could David have a son and call him Lord?  The answer said that by earthly rules, it doesn’t work.  But by heavenly rules, it could work.

            In the quoted Psalm, written by David, the Lord is talking to the Lord.  He is a multi-person, but singular identity.  This Messiah is the son of David, born of the line of David, but more than just a son of a son of a son of a son of David.  In the heavenly realm, this Messiah is the Lord, part of a divine relationship with himself.  In this passage, all three members of the Trinity are mentioned, including the Holy Spirit.