Further notes from the NGSB:
The Unpardonable Sin
Jesus solemn warning about a kind of sin that will not be forgiven, either in this world or the next, is found in three Gospels: Matt. 12:31, 32; Mark 3:2830; Luke 12:10. It is specifically blasphemy against the Spirit. This blasphemy is an act performed by speaking, understood as an expression of the thoughts of the heart (Matt. 12:3337; cf. Rom. 10:9, 10). In the particular context the opponents of Jesus were saying that the Power doing good works among them was not God but the devil. Jesus distinguishes between this blasphemy and other sins, both other sins of speech and other sins in general. As the Bible teaches, God forgave sins of incest, murder, lying, and even Pauls persecution of the church, which Paul did while breathing threats and murder against Gods people (Acts 9:1).
What makes the unpardonable sin different from others is its relation to the Holy Spirit. It is the Holy Spirits work to enlighten the mind of sinners (Eph. 1:17, 18), to reveal and teach the gospel (John 14:26), persuading souls to repent and believe the truth (cf. Acts 7:51). The Spirit not only explains the Word of God, but He opens the mind so that it is perceived (2 Cor. 3:16, 17). When His influence is deliberately and knowingly refused, in opposition to the light, then the irreversible sin can be committed as a voluntary, informed act of malice. In response there is a hardening of the heart from God that rules out repentance and faith (Heb. 3:12, 13). God permits the decision of the human will to be permanent in this case. God does not do this lightly or without cause, but in response to an offense against His love.
A person who wants to repent, that is, to reverse the sins they may be guilty of, has not suffered this hardening and has not committed the profound act of hatred that God has determined He will not forgive. Anyone who has been born again will not commit this sin, because the Spirit lives in that person, and God is not divided against Himself (1 John 3:9).
The other verses dealing with the unpardonable sin are Heb. 6:46; 10:2629; and 1 John 5:16, 17. These show that the possibility of this sin depends on there being particular enlightenment and understanding from God and that it is not a common, everyday matter. Jesus said all sins and whatever blasphemies will be forgiven, excepting only this one sin.
New Geneva study Bible. 1997, c1995 (electronic ed.) (Ge 1:1). Nashville: Thomas Nelson.