There is a friar whose text I read on this topic, Friar Lawrence Farley, who explains it better than I could.What is the unforgivable sin?
>The context of Christ’s declaration reveals it: the sin against the Holy Spirit is the sin of rejecting Christ as a blaspheming deceiver. The Pharisees saw Christ’s miracles and His spectacular exorcisms. They could not deny the reality of the exorcisms; they just said that He could only do such things because He was in league with Satan. “It is only by Beel-zebul, the prince of demons, that this man casts out demons” (Matthew 12:24). (Note in passing their malevolence and hatred of Jesus: they cannot even bring themselves to say His Name. He is “this man”.) This is not just slander against Jesus, but against the Spirit of God Himself, for it declares the Holy Spirit through which Jesus cast out demons (Matthew 12:28) was an unclean spirit. Our Lord’s foes were in fact setting themselves against all that God was doing, rejecting His coming Kingdom as a deception and a fraud. That Kingdom was the only place where grace and forgiveness flowed into the world, so that by continuing to reject the Kingdom, they rejected with it the only source of forgiveness. Blasphemy against the Holy Spirit was not unforgivable in the sense that God refused to forgive those who repented of it, but because in persisting in this sin they cut themselves off from the possibility of forgiveness.