Matthew 12:31 Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Spirit shall not be forgiven unto men. 32 And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Spirit, it shall not be forgiven him, neither in this world, neither in the world to come.
Jesus’ miracles were performed by the power of God.
John 14:10 Believest thou not that I am in the Father, and the Father in me? the words that I speak unto you I speak not of myself: but the Father that dwelleth in me, he doeth the works.
Acts 2:22 Ye men of Israel, hear these words; Jesus of Nazareth, a man approved of God among you by miracles and wonders and signs, which God did by him in the midst of you, as ye yourselves also know:
In context the Pharisees were blaspheming the power of God.
Matthew 12:22 Then was brought unto him one possessed with a devil, blind, and dumb: and he healed him, insomuch that the blind and dumb both spake and saw… 24 But when the Pharisees heard it, they said, This fellow doth not cast out devils, but by Beelzebub the prince of the devils.
The Pharisees should have known that God alone has the power to perform miracles. In the miracle of healing God himself was bearing testimony to his Son. The power of God had performed the miracle, but the Pharisees were committing blasphemy in telling the people that it was the power of a non-existent entity ‘Beelzebub the prince of the devils’. Beelzebub was originally an idol worshipped as a God at the Philistine city of Ekron. King Ahaziah had died because he chose to enquire of the idol Baalzebub rather than the true God.
2 Kings 1:16 And he said unto him, Thus saith the LORD, Forasmuch as thou hast sent messengers to enquire of Baalzebub the god of Ekron, is it not because there is no God in Israel to enquire of his word? therefore thou shalt not come down off that bed on which thou art gone up, but shalt surely die.
By New Testament times the name ‘Beelzebub’ had been assigned to the source of ‘demons’ thought to be the cause of mental illness.
Mark confirms that the blasphemy consisted of saying that the miracle performed by the Holy Spirit was done by ‘an unclean spirit’ associated with an idol.
Mark 3:28 Verily I say unto you, All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme: 29 But he that shall blaspheme against the Holy Spirit hath never forgiveness, but is in danger of eternal damnation: 30 Because they said, He hath an unclean spirit.
I hope you find this helpful.