The Holy Spirit is the Lord who draws us to faith in Christ. Rejecting the wooing of the Holy Spirit is offensive and is the signal that the rejecter is not worthy of Christ.
It is one thing to resist the Holy Spirit. Many have done so and later been wooed by the Holy Spirit and received Christ Jesus as Saviour.
However, to speak against the Holy Spirit is to openly, verbally and orally deny His existence, His purpose or His authority. This is extremely serious because if puts a person on record before the Lord and man.
The Lord honours His Word and He honours our words as well. Mat 10:33
He states that a person's words against Him requires Him to speak against them to the Father. What does that mean? It means that His Holy Spirit will not dispense Grace to the denier and leave them to inevitable destruction.
If Jesus is not a person's advocate before His Holy Spirit they are condemned to the Lake Of Fire Mat 12:32.
Faithfulness is a characteristic of the Lord. If faithfulness is not a part of a person's character they will deny Him eventually. A person's denial of Him will require His denial of them.
Faith in Christ Jesus and Confession of Him as Lord God and Saviour is required to escape Hell and to enter Heaven.
Refusing to Believe on Christ Jesus or to come under His lordship is Unpardonable. The Order that God demands in His Character cannot allow Rebellion in His Prescence.