What does Mark 12:6 mean?

Jesus' use of this parable is meant to turn the religious leaders' arrogance against themselves. At first, they might have thought of themselves as the parable's landowner. It would have quickly dawned on them that Jesus implies they are actually the abusive tenants, and God's prophets are the servants. That's confrontational enough—but this turn in the story would have been outrageous and alarming to the religious leaders. Jesus is saying that He is the son of the landowner: He is the Son of God.

In the Old Testament, "son of God" usually refers to angels (Job 1:6), the Nephilim from Genesis 6:2–4, or the Israelites in general (Exodus 4:22). Jews don't see it as a specifically Messianic term, although it is used loosely for David and his dynasty (2 Samuel 7:14; Psalm 89:27–28). Apocryphal literature does refer to the Messiah as the son/Son of God, but also uses that title for those who follow God closely. This kind of imagery is standard in Judaism. When the religious leaders call themselves the sons or children of Abraham, Jesus counters that they are not the children of God, but the children of the Devil (John 8:39–44). Their actions, particularly their rejection of Jesus, show who it is they resemble.

As used here, however, the phrase "beloved son" clears away all the historical metaphor. It pushes Jesus' claims into an area of theology for which the Jews do not yet have a name. Jesus is saying that the God of the universe, who is one, has a singular son. Years later, church leaders will wrestle with the concept of the Trinity, but on this day, Jesus' audience must be either very confused or very incensed that He presumes to state God has a son.