How did theologians conclude that blasphemy of the Holy Spirit meant a refusal for repentance?

The task of a theologian is to bring together the many parts of scripture into a consistent whole. Passages may appear to be contradictory, at least on the surface, and so the theologian seeks an understanding of both passages together, to identify the intention of the one divine author of God of both passages. Some passages may initially appear very simple, but become more complex when we read them in the light of the rest of the scriptures.

When we think about the Bible's general teachings on salvation it's clear that God forgives and saves sinners guilty of great and heinous sins: Moses who murdered the Egyptian guard. David who committed adultery (and many would say with a woman who could not consent) and then murder to cover it up. Peter who disowned knowing Jesus himself. Paul who persecuted and murdered the Church itself. If God can forgive lies, adultery, persecution, and murder, then it would be rather odd for mere verbal blasphemy to be unforgivable. Most Christians would see the whole Bible as indicating that all sins can be forgiven of the sinner who genuinely repents and turns to God in faith. That leads us to think that Jesus must have meant something else other than literal verbal blasphemy.

Well what can't be forgiven? Most Christians would say that any specific sin can be forgiven. The people we (not counting Christian universalists) know for certain won't be forgiven are those who consciously and deliberately reject God's offer and call of salvation. So this is why many say that the "unforgivable sin" is the sin of unrepentance.

Why does Jesus highlight the Holy Spirit in this passage? I think it is because of the particular role of the Spirit in salvation. While all persons are involved in salvation, they each have a particular focus: the Father whose masterplan it is to send the Son for our sake, the Son who died and rose for us, and the Spirit who in particular applies the Gospel to us as individuals. It is the Spirit who convicts us of our sin and who witnesses within us to the truth of the Gospel and the worthiness of Christ to receive the worship of our lives. So this may be why blasphemy against the Father and Son can be forgiven, for their role in salvation is a step removed, while the Spirit's role is within our souls. We may temporarily reject the Father and the Son, but what brings eternal death is to reject the Spirit when he calls us to repent. If we say that the Spirit's attempts to convict us of sin and call us to righteousness are from the devil, then we will want no part of the Gospel.

Would this understanding make sense of the context of Matthew 12? What has happened here is that the Pharisees have seen Jesus free a demon possessed man, and instead of praising God, they say that it was a demon-vs-demon struggle, with Jesus being able to free the man only because he serves a more power demon than the one who had possessed the man. While we think of the witness of the Holy Spirit primarily as an internal witness, at this time during the life of Jesus, the Spirit was also witnessing to the nation through the acts of Jesus, acts like casting out demons. Those who witnessed Jesus heal this man should have recognised that God was powerfully at work here to accomplish good, and that's what most people did, as they then ask whether Jesus is the son of David (a messianic reference.) But the Pharisees refuse to see the good of this act, and insist it is the work of Beelzebul. Their outward blasphemy - calling the Holy Spirit who indwells Jesus the prince of demons - is the expression of their inward stubborn unrepentance, for only someone who has so thoroughly blinded themselves to the ways of God could mistake God's loving miraculous acts for the acts of Satan. When Jesus says blasphemy against the Holy Spirit cannot be forgiven I don't think he means that any slander against the Spirit irreversibly damns someone, but that these Pharisees have just demonstrated that they are totally opposed to listening to God to the very core of their being.