Curious about our Lord's teaching on the non-forgiveness of sins against the Holy Spirit? When Jesus says: "Therefore I tell you, every sin and blasphemy will be forgiven, but the blasphemy against the Spirit will not be forgiven. And whoever says a word against the Son of man will be forgiven; but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come." (cf Mt 12:31-32) Similar words appear in Mark 3:28-30 and Luke 12:10. If the three Divine Persons are perfectly equal, even though distinct one from another, why does Our Lord say that sins against the Holy Spirit cannot be forgiven whereas sins against the Son can?
We should begin by recalling that God wants all to be saved and to come to the knowledge of the truth, (cf. 1 Tim 2:4) He does not want anyone to be lost, but calls all to repentance, (cf. 2 Pet 3:9) The Redemption Christ won for us is superabundant and atones for all sins of all people, (cf. Rm 5:12-21)
Christ gave his Church the power to forgive sins by means of the sacraments of Baptism and Reconciliation. This power is unlimited, in the sense that the Church can forgive all sins of all the baptized as often as they confess their sins with true sorrow. This teaching is a dogma of faith, proclaimed in the Council of Trent. (Read more)
So, are there any sins that cannot be forgiven? First, we must put Our Lord's words into context. He said to them right after the Pharisees accused him of casting out demons with the power of Beelzebul, the prince of devils (cf. Mt 12:24) "...but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin - for they had said, 'He has an unclean spirit,'" says St Mark. (cf Mk 3:30)
The Pharisees had attributed to Satan, the prince of devils, the good works done by God himself. The sin is called blasphemy against the Holy Spirit because external manifestations of God's goodness are especially attributed to the Blessed Trinity's third person.
Anyone who commits this sin is like a sick person who has lost faith in his doctor to the point where he rejects him as an enemy and regards the medicine that can save his life as poison. It is not that God cannot or will not forgive the sin, but rather that the person, in his blindness, rejects Jesus Christ, his teaching, and miracles, and despises the Holy Spirit's graces as if they came from the devil and were intended to trap him. (cf the Roman Catechism, II.)
Such a person does not seek forgiveness because he has lost faith in God's power and mercy. Thus, the sin is unforgivable not because of its gravity or malice - there is no sin so grave that it cannot be forgiven if the sinner repents and asks for forgiveness - but because the sinner's disposition is such that he closes the door and does not ask to be forgiven.
In a nutshell, sin against the Holy Spirit is the refusal to repent and seek forgiveness. It is the sinner, not God, who closes the door to forgiveness by refusing to repent.
The Catechism of the Catholic Church explains it like this: "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. Such heart hardening can result in final impenitence and eternal loss. (CCC 1864)
It is critical to remember God's mercy and to remind those who have committed serious sins that they can always be forgiven. All they have to do is ask for forgiveness and resolve to work hard to avoid committing the same sins again.