What is the sin against the Holy Spirit?
Mark 3:23 – 30
23 And he called them to him and said to them in parables, “How can Satan cast out Satan? 24 If a kingdom is divided against itself, that kingdom cannot stand. 25 And if a house is divided against itself, that house will not be able to stand. 26 And if Satan has risen up against himself and is divided, he cannot stand, but is coming to an end. 27 But no one can enter a strong man’s house and plunder his goods, unless he first binds the strong man. Then indeed he may plunder his house.
28 “Truly, I say to you, all sins will be forgiven the children of man, and whatever blasphemies they utter, 29 but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin”— 30 for they were saying, “He has an unclean spirit.”
Catechism of the Catholic Church
1864″Therefore I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven.”136There are no limits to the mercy of God, but anyone who deliberately refuses to accept his mercy by repenting, rejects the forgiveness of his sins and the salvation offered by the Holy Spirit.137Such hardness of heart can lead to final impenitence and eternal loss.
136 -Mt12:31; cf.Mk3:29;Lk12:10.
137 – Cf. John Paul II,DeV46.
Aquinas: SMT SS Q A Body Para. 2/4
Augustine, however, says that blasphemy or the sin against the Holy Ghost, is final impenitence when, namely, a man perseveres in mortal sin until death, and that it is not confined to utterance by word of mouth, but extends to words in thought and deed, not to one word only, but to many. Now this word, in this sense, is said to be uttered against the Holy Ghost, because it is contrary to the remission of sins, which is the work of the Holy Ghost, Who is the charity both of the Father and of the Son. Nor did Our Lord say this to the Jews, as though they had sinned against the Holy Ghost, since they were not yet guilty of final impenitence, but He warned them, lest by similar utterances they should come to sin against the Holy Ghost: and it is in this sense that we are to understand Mark 3:29,30, where after Our Lord had said: “But he that shall blaspheme against the Holy Ghost,” etc. the Evangelist adds, “because they said: He hath an unclean spirit.”