In Matthew 12:22-32 (and Mark 3:22-30), Jesus explains the unpardonable sin—the one sin that God will not forgive:
Is there an unpardonable sin? What is it?
"Therefore I tell you, every sin and blasphemy will be forgiven people, but the blasphemy against the Spirit will not be forgiven. And whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come" (Matthew 12:31-32).
This passage has caused many people a great amount of confusion. Some brazenly speak against the Holy Spirit as an act of rebellion against God, believing this will keep them out of heaven forever. Others are fearful that, in a moment of weakness or ignorance, they ruined all chances of reconciliation with God. This isn't what blasphemy against the Holy Spirit means.
We who live after the time of Christ cannot blaspheme the Holy Spirit in the way Jesus' contemporaries could. Jesus was referring to people who directly saw the miracles He performed and then asserted His power and mission were from Satan. They attributed the works and will of the Holy Spirit in Jesus' ministry to Satan—in a circumstance that made it absolutely clear that it was the Holy Spirit at work. This is unpardonable. The people (in this case, the Pharisees) who verbally expressed their belief that Jesus was working for Satan were publicly rejecting the Holy Spirit.
We cannot commit the unpardonable sin, although we can die in an unpardonable state. The unpardonable state is to die having not accepted Jesus' gift of salvation. Since Jesus is the only way to be saved (Romans 6:23), and there is no offer of salvation after death (Hebrews 9:27), the state in which we die is the state which we will inhabit forever—it cannot change or be pardoned.
While we are living, there is no sin we can commit, no action we can perform, and no phrase we can utter that is unpardonable. Attributing the works of Christ to Satan is not unpardonable for us because we were not there in Jesus' presence, faced with incontrovertible proof. Suicide is not unpardonable—our salvation is based on the Holy Spirit in us, not the method of our death. It is not even unpardonable to live a life of violence, rejecting Christ at every step. "If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness" (1 John 1:9). There is nothing God will not forgive if we ask Him to.
There is a danger in not understanding the unpardonable sin. Not just for the fools who think they can utter a phrase and thwart the power of God, but for the seeking who fear they have lost their chance of salvation. In both cases, it's important to remember that God is bigger than any sin we can commit. We cannot commit the unpardonable sin.
Blasphemy against the Holy Spirit - What is it?
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How can I know what is a sin and what isn't?
What are the seven deadly sins?
How can I be saved?
Truth about Sin