"I tell you the truth, all sin and blasphemy can be forgiven, but anyone who blasphemes the Holy Spirit will never be forgiven. This is a sin with eternal consequences."
This passage from the Gospel of Mark has worried Christians for centuries. It speaks of a sin that will not be forgiven, often called the "unpardonable sin." As a pastor, I have counseled with people who feared that they had committed this sin and that God would never forgive them. Today I'll share with you what I have said to them.
Mark 3:28-29 comes in a passage that describes various criticisms of Jesus. His family, for example, thought he was out of his mind (3:21). The religious teachers from Jerusalem offered another explanation for Jesus's behavior: "He's possessed by Satan" (3:22, literally, "He has Beelzebul").
Jesus countered this attack in two ways. First, he explained that it made no sense if he were casting out demons with demonic power (3:23-27). Logically, the power of Jesus must have been opposed to and greater than the power of Satan. Second, Jesus delivered the bad news, namely, that someone who "blasphemes the Holy Spirit will never be forgiven" (3:29). Most narrowly, this referred to the accusations of the Jewish leaders, who attributed the work of God to Satan.
Blaspheming the Holy Spirit is not a matter of a mistaken comment here or there. Rather, it is persistent, intentional rejection of God's work and God's grace. It isn't just attributing the work of the Spirit to Satan, however. It is also choosing to resist that work in one's own life. To put it most bluntly, the unpardonable sin is rejecting the very grace that pardons. It is saying "No" to God's offer of forgiveness.
In my experience, those who worry about the unpardonable sin are not anywhere close to committing it. If anything, what they lack is not openness to God's forgiveness, but rather an ability to believe the good news of Mark 3:28, that "all sin and blasphemy can be forgiven."
If you will only be open to God's work in your life, God will forgive. Remember the encouragement of 1 John 1:9: "[I]f we confess our sins to him, he is faithful and just to forgive us our sins and to cleanse us from all wickedness." From all wickedness!