The Pharisees blasphemed the Holy Spirit, because their self-made religion had taken them so far away from God’s religion that they did not recognize the work of the Spirit when He was right in their midst, working through Jesus. So then, He announced,

“Therefore I say to you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven men” (Matthew 12.31).

Jesus certainly did not minimize all other sins and blasphemies, but when someone calls the Holy Spirit, Beelzebub, that person shows he is so far gone into sin, that he will never come out of it, hence, will never be forgiven.