The Enchiridion 83
83 The Man Who Despises the Mercy of God is Guilty of the Sin Against the Holy Ghost PL 40, 272
Now the man who, not believing that sins are remitted in the Church, despises this great gift of God’s mercy, and persists to the last day of his life in his obstinacy of heart, is guilty of the unpardonable sin against the Holy Ghost, in whom Christ forgives sins. But this difficult question I have discussed as clearly as I could in a book devoted exclusively to this one point.
Peccatum in Spiritum sanctum. Qui vero in Ecclesia remitti peccata non credens, contemnit tantam divini muneris largitatem, et in hac obstinatione mentis diem claudit extremum, reus est illo irremissibili peccato in Spiritum sanctum, in quo Christus peccata dimittit. De qua quaestione difficili in quodam propter hoc solum conscripto libello enucleatissime, quantum potui, disputavi.