31 “Therefore I say to you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven men.
32 Anyone who speaks a word against the Son of Man, it will be forgiven him; but whoever speaks against the Holy Spirit, it will not be forgiven him, either in this age or in the age to come.
There are several interpretations of these verses. Some commentaries attempt to apply the passage to people today and say the sin against the Holy Spirit is not trusting Christ for salvation. Walvoord takes a view very like this:
There has been much misunderstanding about blasphemy against the Holy Spirit. Here it is properly defined as attributing to Satan what is accomplished by the power of God. Such a sin is not unpardonable in itself, but rather because it rejects the person and work of the Holy Spirit, without whom repentance and restoration are impossible. As far as it applies today, it is not the thought that one seeking pardon will not find it, but rather that one who rejects the Holy Spirit will not seek pardon. — Walvoord, page 89
That seems like semantic juggling to me. Gaebelien takes another view:
They had blasphemed the Spirit, spoken injuriously about Him, in saying that Beelzebub, the prince of demons, was present with Christ and not the Holy Spirit. This they did maliciously. And this and nothing else is the sin of which our Lord here speaks. The sin is to charge the Lord with doing His miracles through Satanic power and not through the Holy Spirit. We do believe, therefore, that this sin could only be committed as long as our Lord Jesus Christ was in the earth and that it was committed by the Pharisees with their blasphemies. — Gaebelein, page 250.
Morgan takes the stand that the Lord’s words weren’t a sentence, but a warning.
Pentecost takes yet another view.
Christ was warning that generation in Israel that if they rejected the Father’s testimony and the Spirit’s testimony to His person and His work, there was to be no further evidence that could be given. Their sins would stand unforgiven and result in temporal judgment on that generation. That judgment ultimately fell in A.D. 70 when Jerusalem was destroyed. This sin, then, was not viewed as the sin of an individual but rather as the sin of the nation, and this sin brought that whole generation under divine judgment. — Pentecost, page 207.
What I’ve heard, from Stam, I think (although I can’t put my hands on it right now) is this: The generation of Israel on the earth at that time had already rejected God the Father in the Old Testament, law period which was coming to a close. They had just rejected God the Son by attributing His miracles to Satan. They would get a chance to accept the testimony of God the Spirit — as evidenced by Christ’s miracles, but finally at Pentecost. If they rejected the Spirit, there was no hope left because they would have rejected the Father, the Son and the Spirit.
If this is the true meaning, the Lord’s words function as a warning along the lines of “Be careful. You just swung and missed and now have strike two against you and the next pitch is on the way.”
So, this sin could only be committed by that generation, the only one that existed during the Old Testament law period, the period of Christ’s ministry on earth and the Holy Spirit’s ministry at Pentecost.