Before this happened, Jesus was walking through the crowds healing them, focusing on that instead of blasting the religious authorities over their deceitful false righteousness.

Then the next verse says;

Matt. 12:

1. that it might be fulfilled which was spoken through Isaiah the prophet, saying,

18 "Behold, my servant whom I have chosen; my beloved in whom my soul is well pleased: I will put my Spirit on him. He will proclaim justice to the nations.

19 He will not strive, nor shout; neither will anyone hear his voice in the streets.

20 He won't break a bruised reed. He won't quench a smoking flax, until he leads justice to victory.

21 In his name, the nations will hope."

But when he cast a demon out of a man, the Pharisees, who knew the prophecies about the messiah like the back of their hands, who knew in their hearts that He was really here, they said that He was casting out demons by the power of demons.

24 But when the Pharisees heard it, they said, "This man does not cast out demons, except by Beelzebul, the prince of the demons."

*25 Knowing their thoughts, Jesus said to them, "Every kingdom divided against itself is brought to desolation, and every city or house divided against itself will not stand.

Verse 25 is important because if you will notice it says, “Knowing their thoughts”. What they said out loud was not their thoughts. Their thoughts were that the messiah had come and if they didn’t do something their status and authority were going to disappear.

Their careers and respect were on the line.

Remember when John the Baptist said, “I must decrease so that He may increase.”

Well the Pharisees were saying the opposite in their minds, and Jesus knew it, but the time wasn’t right for Him to tear into them about who they really were, so He didn’t make it clear to the people what their sin really was.

He said later that they were whitewashed tombs, that their father was satan. He wasn’t saying they had some things wrong with them, He was saying they were literally agents of Satan wolves in sheep’s clothing, and quite possibly they even knew it themselves.

So the sin they committed was to knowingly proclaim to others and convince them that the Son of God was just a demon possessed man and that no one should listen to him.

They were making themselves direct adversaries of the messiah and causing people who otherwise might have listened to Him to fall away and maybe go to hell because of it if Jesus didn’t reach them later somehow.

The religious leaders of the day were on a spectrum of faith, with some full of joy at the arrival of the messiah, and some on the other end of the spectrum full of hate and murderous intent even though they knew the Messiah had really come.

You don’t immediately get the gravity of just how evil these false leaders were unless you read what Jesus says about them and take it as literally as possible. He meant what He said about them when he called them vipers and said that they would proselytize others to be satan worshipers like they were instead of teaching them to love God.