Catechism of the Catholic Church §1864 — excerpt
"Whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin." There 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.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: John Paul II, DeV 46