"I have come to set the earth on fire, and how I wish it were already blazing! There is a baptism with which I must be baptized, and how great is my anguish until it is accomplished! Do you think that I have come to establish peace on the earth? No, I tell you, but rather division. From now on a household of five will be divided, three against two and two against three; 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." (Lk 12:49-53)
There is a crisis on the horizon. It poses a challenge to loyalty. Even the good news preached by Jesus is at question. Jesus was preaching of peace unity etc. The up coming crisis is a challenge to this preaching. The message of Jesus is cutting across the family and dividing the family in to groups.
The saying of Jesus in Lk 12:52-53 seems to reflect the prophecy of Micah 7:6 (For the son dishonors his father, the daughter rises up against her mother, the daughter-in-law against her mother-in-law, and a man's enemies are those of his household.).
The crisis that is coming up will disturb his own central mission. That is why he is speaking about a Baptism and the anguish to accomplish it.
