49 “I have come to set the earth on fire, and how I wish it were already blazing! 50 There is a baptism with which I must be baptized, and how great is my anguish until it is accomplished! 51 Do you think that I have come to establish peace on the earth? No, I tell you, but rather division. 52 From now on a household of five will be divided, three against two and two against three; 53 a father will be divided against his son and a son against his father, a mother against her daughter and a daughter against her mother, a mother-in-law against her daughter-in-law and a daughter-in-law against her mother-in-law.” (Luke 12:49-53)
Throughout Luke 12 Jesus has continued to call for people to “see,” a message that has been present since the beginning of the mission of the 72 others in the beginning of Luke 10. A message made clear upon their return: “Blessed are the eyes that see what you see. For I say to you, many prophets and kings desired to see what you see, but did not see it, and to hear what you hear, but did not hear it.” (Luke 10:23-24). Along the continuing journey to Jerusalem each person becomes an opportunity for Jesus to help them (and the crowds) to see more clearly, more richly: the scholar of the Law in Luke 10:25 ff; Martha and Mary (vv.38-42); the disciples in Luke 11, as well as the Pharisees in that same encounter; and Jesus continually speaks so that they will become “rich in what matters to God” (Luke 12:21).
There is an irony that runs throughout this passage: while the Kingdom’s arrival is meant to usher in an era of peace and unity, its announcement is the initiator of division. Divisive because it requires decision and commitment in the face of a coming judgment (Luke 12:4-10), clearly told in the parable of the Rich Fool (vv.16-21). In vv. 49-53 the Word of God is a refining and purifying fire. As Culpepper  notes, “The announcement was foreshadowed by Luke 3:16, where John claims that Jesus will baptize with the Holy Spirit and with fire. In the same context, fire is used as an image of God’s judgment (3:9, 17; cf. 12:49; 17:29). Ironically, when the fire comes upon the disciples in Acts, it is not the consuming fire of judgment but the purifying fire of the Holy Spirit. Nevertheless, the crisis of judgment is never far away (see Mal 3:2–3). A non-canonical saying relates fire to Jesus’ mission: ‘Whoever is near me is near fire; whoever is distant from me is distant from the kingdom’ (Gospel of Thomas 82). With the coming of the Spirit, the work of the church, and the approach of the kingdom, division and strife would be intensified. Jesus is saying, ‘Let it start now!’”
Alan Culpepper, Luke in The New Interpreter’s Bible, Vol. IX (Nashville, TN: Abingdon Press, 1995)