All of the challenging words and images in the Sunday gospel (Luke 12:48-53) are with a purpose in mind: Jesus’ ministry necessarily demanded a definitive decision on the part of his would-be followers. Their yes had to be yes; their no had to be no. Christianity precludes indecision as well as compromise. Surrendering to compromise and infidelity was the error made by so many of the Old Testament communities – even when confronted by their prophets. When the prophet dared to confront them, they turned on the prophet, denounced him and often tried to kill him. Surrendering to lukewarmness in their discipleship was also the mistake made by the church in Laodicea; a community that some described as a perfect model of inoffensive Christianity. Therefore, they were told that God would spew them out as one would spew out bitter, tepid water. 

Culpepper (Luke, New Interpreters Bible, p.267) reflects on this passage:

Repeatedly, the warnings about the coming judgment have forced us to examine the implications of our commitments. It is all too easy to make commitments in one area of life as though they did not affect other areas also. Jesus warned that those who make a commitment to him will be persecuted, that a commitment of faith also means that our attitude toward material possessions must change, and that moral responsibilities must be taken with even greater seriousness. Now Jesus warns that persons who make a commitment to him will find their relationships to others, even those closest to them, affected by that commitment. We cannot make a commitment to Jesus Christ as Lord without its affecting the way we related to friends and to family members. Because our commitment to Christ shapes our values, priorities, goals, and behavior, it also forces us to change old patterns of life, and these changes may precipitate crises in significant relationships. 

Jesus is asking his disciples to clearly see the culmination of his mission in the return of the Son of Man at the time of judgment. He is already engaged in the task of lighting a fire on the earth to prepare people for judgment, all the while noting that judgment is taking place as people decide for or against him. 

The “fire” of Christ is also given in the Holy Spirit as well (Acts 2:3–4); the fire of the Holy Spirit will be cast on the earth through the fulfillment of the events for which Jesus is heading toward Jerusalem. Jesus means by his “baptism” the plunge into this saving mission, a prospect that produces mixed emotions because of the suffering connected with it. Some of his teaching on forgiveness and peace may have given the impression that he was spreading a soft gospel; John the Baptist seems to have worried about that (7:18–23). Jesus assures his listeners that Christian discipleship is costly, even causing division in the family (Mark 3:21; John 7:5).