What does Matthew 21:43 mean?

This states, directly, what the parable of the tenants in the vineyard said indirectly (Matthew 21:33–41). Christ tells the chief priests and Pharisees standing in front of Him in the temple that the kingdom of God will be taken from them.

Israel's leaders had held a powerful position on earth for centuries. Israel was God's chosen people, and He dealt with His people most directly through the leaders who were meant to guide them to obedience and righteousness and trust in Him. Those leaders, though, had failed for far too long, rejecting God's prophetic messengers (Acts 7:52). Now they had even rejected God's own Son, Jesus, as the Messiah. The result would be that the leaders of Israel would lose the role of tending the vineyard of God's people on earth.

Instead, Jesus says, the kingdom of God will be given to a people who will produce the fruit of righteousness and obedience to Him. Commentators disagree on whether Jesus is referring explicitly to moving the focus of God's relationship with humanity from Israel to the church, made up of all who trust in Jesus alone for their salvation. The church begins with the twelve apostles and grows for a time as an apparent extension of Judaism before eventually becoming a mostly Gentile movement. What all agree on is that Jesus will now be the dividing line between those who are the people of God and those who are not. All who would belong to God must come through Jesus, the cornerstone.