The idea of “blasphemy against the Holy Spirit” comes from the Gospels, where Jesus calls this a sin for which there is no forgiveness (Luke 12:10, cf. Matthew 12:32; Mark 3:29). Unfortunately, Jesus does not spell out definitively what blasphemy against the Holy Spirit means. It would have cleared things up if he had said, “Whoever says these specific words or does this specific act, they sure are in for it. No grace for them.” Alas, he did not.
I know many who fear they have committed this blasphemy and thus forfeited their opportunity to be saved. When asked what people are worried about, they will usually mention some combination of rejecting the Spirit’s urging, saying something disrespectful about the Holy Spirit, or committing some particularly heinous sin.
This isn’t just idle speculation for me, either. At one point in my past, I thought that I had committed the unpardonable sin. I thought since I had seen so much of God’s grace that when I turned away and sinned so egregiously, surely I had committed that blasphemy.
In this case, the popular pastoral advice is spot on: If you are worried that you have committed that blasphemy, you probably haven’t. The idea of blasphemy against the Holy Spirit includes the removal of any desire to be reconciled to Jesus. It is God, after all, who puts in us the desire to come to him (Philippians 2:12-13). Thus, your fear about having reached the “point of no return” is good proof that you haven’t. If you want to repent, he will always receive you. He will not cast out, for any reason, any who come to him (John 6:37).
So what is blasphemy against the Holy Spirit? Ed Welch gives a biblical and compassionate answer to that question here. I would summarize some of what he says like this: Blasphemy against the Spirit is any hardened and consistent stance against God. We all have moments, even sustained periods of our lives, when we defy God. But there may come a point in a person’s life when that becomes an unyielding defiance against God and he or she refuses to let the Spirit speak. At that point, God finally says, “You want to do this on your own? You want me out of your life? Okay.” Or to put it another way, the only unpardonable sin is refusing to let God pardon you.