Jesus Christ was prophesizing the severe punishment Israel was going to face which happened during Jewish Wars (66-70 AD).
The generation of Jews during the time of Jesus was extremely evil.
We see Jesus calling them "wicked (or sinful) and adulterous generation." (Matthew 12:39, Matthew 16:4, Mark 8:38, etc.). We also see Jesus calling people in that generation that their father is devil (John 8:44) due to their evil nature. We also see Both Jesus Christ and John the Baptist calling them "offspring of vipers" (Matthew 23, Luke 3).
Even Jewish Priest Josephus agrees with Jesus on the wickedness of that generation.
"I shall therefore speak my mind here at once briefly: — that neither did any other city suffer such miseries, nor did any age ever breed a generation more fruitful in wickedness than this was, from the beginning of the world." (Jewish Wars V, 10:5).
We all know that Jesus Christ came to forgive those who repent. But Jesus Christ also came to judge those who didn't repent.
Luke 12:51-53 can also be read in Matthew 10:34-36.
Matthew 10:34-36 (NIV) - "Do not suppose that I have come to bring peace to the earth. I did not come to bring peace, but a sword. For I have come to turn “‘a man against his father, a daughter against her mother, a daughter-in-law against her mother-in-law— a man’s enemies will be the members of his own household."
"A man's enemies will be the members of his own household" - This can also be read when Jesus Christ prophesized about the destruction of the city of Jerusalem and the city of temple of Jerusalem in Mark 13, Luke 21.
Mark 13:12-13 (NIV) - "Brother will betray brother to death, and a father his child. Children will rebel against their parents and have them put to death. Everyone will hate you because of me, but the one who stands firm to the end will be saved."
Luke 21:16-19 (NIV) - "You will be betrayed even by parents, brothers and sisters, relatives and friends, and they will put some of you to death. Everyone will hate you because of me. But not a hair of your head will perish. Stand firm, and you will win life."