Attributing a demonic spirit to Christ.
The sin against the Holy Spirit is claiming that the works of God are the works of Beelzebub.
It is calling "demonic" what is "godly".
The passage is very interesting. Jesus healed a sick person, but this GOOD deed had been adjudicated to the works of the demon. Please read the text, including the immediate corollary: Jesus explaning that a tree is known by its fruit.
Then one possessed with a demon was brought to Him, blind and mute, and He healed him, so that the blind and mute man both spoke and saw. 23 All the people were amazed and said, “Is He not the Son of David?”
24 But when the Pharisees heard it, they said, “This Man does not cast out demons, except by Beelzebub the ruler of the demons.”
25 Jesus knew their thoughts and said to them, “Every kingdom divided against itself is brought to desolation. And every city or house divided against itself will not stand. 26 If Satan casts out Satan, he is divided against himself. Then how will his kingdom stand? 27 And if I cast out demons by Beelzebub, by whom do your sons cast them out? Therefore, they shall be your judges. 28 But if I cast out demons by the Spirit of God, then the kingdom of God has come upon you.
29 “Or else how can one enter a strong man’s house and plunder his goods unless he first binds the strong man? And then he will plunder his house.
30 “He who is not with Me is against Me, and he who does not gather with Me scatters abroad. 31 Therefore I say to you, all kinds of sin and blasphemy will be forgiven men, but the blasphemy against the Holy Spirit will not be forgiven men. 32 Whoever speaks a word against the Son of Man will be forgiven. But whoever speaks against the Holy Spirit will not be forgiven, neither in this world, nor in the world to come.
33 “Either make the tree good and its fruit good, or else make the tree corrupt and its fruit corrupt. For the tree is known by its fruit. 34 O generation of vipers, how can you, being evil, speak good things? For out of the abundance of the heart the mouth speaks. 35 A good man out of the good treasure of his heart brings forth good things. And an evil man out of the evil treasure brings forth evil things. 36 But I say to you that for every idle word that men speak, they will give an account on the Day of Judgment. 37 For by your words you will be justified, and by your words you will be condemned.”