I always wondered why blasphemy against the Holy Spirit was the only unforgivable sin. This makes it crystal clear.
Jesus has just worked a miracle but the scribes refuse to recognize it "for they had said 'He has an unclean spirit'" (v. 30). They do not want to admit that God is the author of the miracle. In this attribute lies the special gravity of blasphemy against the Holy Spirit -- attributing to the prince of evil, to Satan, the good works performed by God himself ... That is why our Lord says that he who blasphemes against the Holy Spirit will not be forgiven: not because God cannot forgive all sins, but because that person, in his blindness towards God, rejects Jesus Christ, his teaching and his miracles, and despises the graces of the Holy Spirit as if they were designed to trap him (cf. St. Pius V Catechism, II, 5, 19; St. Thomas Aquinas, Summa theologiae, II-II, q. 14, a. 3). CF. note on Mt 12:31-32.