What does Luke 12:51 mean?

During this lesson, Jesus has spoken about the futility of valuing the things of the earth. He encourages His followers to be faithful, complete their assigned tasks, and wait for Him. Peter, probably speaking for all the disciples, has reminded Jesus that He promised them an honor that the crowd of people following them will not attain to. Jesus agrees, but He knows the disciples don't really understand what is at stake. Yes, the kingdom of God will arrive, but not before Jesus is crucified and the flames of judgment purify the earth (Luke 12:1–50).

His followers will face their own sacrifice. The point of judgment is to determine the value of things and respond accordingly. In the case of the kingdom of God, this means separating the people in the kingdom from those who are not and will not be in the kingdom. Some will accept forgiveness from sins and reconciliation with God through the sacrifice of Jesus and some won't. Those two camps will be divided.

Jesus goes on to make things more personal. Even families will be divided. Some of His disciples may see their own fathers, mothers, sons, and daughters reject Christ. Yes, they will judge Israel on twelve thrones (Matthew 19:28), but not everyone they love on earth will be in the Israel that inherits God's promises in the millennial kingdom (Luke 12:52–53).

This is why they need to be diligent in their work while Christ is away. Everyone will die physically, whether good or evil, and there comes a point when it is fruitless to try to save your own life (Luke 12:4–6). It is repentance that leads to salvation, not good works. But God is patient; every delay of Jesus' return is another chance for people to come to Him for forgiveness (Luke 13:1–9; 2 Peter 3:1–13). That is why Jesus-followers also need to be patient, like God, "not wishing that any should perish, but that all should reach repentance" (2 Peter 3:9)