The Enchiridion 83The Man Who Despises the Mercy of God is Guilty of the Sin Against the Holy Ghost.

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.