What does Mark 12:36 mean?
In addition to being a shepherd, mighty warrior, king, and a man after God's own heart, David was also a poet. He wrote at least seventy-three of the psalms. They include psalms of lament, thanksgiving, and imprecation or request that someone be judged. It's not clear if David knew that many of his psalms are also prophecies, but Psalm 110 certainly is.Psalm 110 begins with this verse that Jesus quotes. The first "Lord" in Jesus' quote is printed as "LORD" in the psalm, which translates to Yahweh, or God. The second title, "my Lord" is the Hebrew Adonai, used for an earthly authority, or as a reverent title for God. The psalm goes on to say that God will expand "the Lord's" kingdom. The Lord will rule despite being surrounded by enemies, and His people will follow Him willingly. The Lord will also be a priest in the order of Melchizedek which outranks the priests from the tribe of Levi (Hebrews 7:1–10).
All this to mean, the figure who will fulfill this prophecy is called "my Lord" by David, despite being David's descendant. While Jesus is the physical descendant (Luke 3:23–31) and legal descendant (Matthew 1:6–16) of David and will rule Israel in a similar manner, He is not solely defined by this subordinate relationship to David. Contrary to cultural assumptions of the time, David's "son" outranks David.
He does so because He is not only David's son. Much as Jesus is a priest of the order of Melchizedek, and therefore has a claim to priestly authority over Aaron's descendants (Hebrews 7), Jesus receives the authority to reign as king from God, not David.