Jesus asks this to correct a natural assumption: that the Messiah would bring outward peace right away. Gill says some supposed He came "to set up a temporal kingdom, in great pomp, and outward peace and tranquility." Jesus wants His disciples to expect something different. Gill explains that Christ came to make peace with God and to give "spiritual and eternal peace," but "not external peace," especially peace that would sacrifice the truth.

Jesus answers at once: "I tell you, Nay; but rather division." He explains: "from henceforth there shall be five in one house divided, three against two, and two against three" (Luke 12:52). The division would reach the closest bonds: father and son, mother and daughter, mother in law and daughter in law (Luke 12:53).

The word "division" is diamerismos. Thayer defines it as disunion or dissension, the opposite of peace, and it appears only here in the New Testament.

In Matthew's parallel, Jesus says, "I came not to send peace, but a sword" (Matthew 10:34), and adds, "a man's foes shall be they of his own household" (Matthew 10:36). Barnes, in his note there, says these words come from Micah 7:6. He stresses that Jesus is still the Prince of Peace and that division is "one of the effects of his coming," not its aim: "The wickedness of men, and not the religion of the gospel, is the cause" of the hostility.

Jesus is honest about what following Him will cost. The gospel is good news, but it calls for a decision, and people do not all decide the same way. Gill says the gospel "sets men apart from one another, even the nearest relations." Jesus' question prepares His disciples so they will not be shaken when it happens.

This fits the whole chapter. Jesus has told His friends not to fear those who can kill the body (Luke 12:4), that the Holy Ghost will teach them what to say before rulers (Luke 12:11-12), and that their Father will care for their needs (Luke 12:30-32). He tells them about division so they can face it with trust rather than surprise.