In Christian hamartiology, eternal sins, unforgivable sins, unpardonable sins, or ultimate sins are sins which will not be forgiven by God. One eternal or unforgivable sin (blasphemy against the Holy Spirit), also known as the sin unto death, is specified in several passages of the Synoptic Gospels, including Mark 3:28–29, Matthew 12:31–32, and Luke 12:10, as well as other New Testament passages including Hebrews 6:4–6, Hebrews 10:26–31, and 1 John 5:16.
The unforgivable sin is interpreted by Christian theologians in unique ways, although they generally agree that one who has committed the sin is no longer able to repent, so one who is fearful that they have committed it has not done so.
Matthew 12:30: “Whoever is not with me is against me, and whoever does not gather with me scatters. Therefore I tell you, people will be forgiven for every sin and blasphemy, but blasphemy against the Spirit will not be forgiven. Whoever speaks 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.”
Mark 3:28–30: “Truly I tell you, people will be forgiven for their sins and whatever blasphemies they utter; but whoever blasphemes against the Holy Spirit can never have forgiveness, but is guilty of an eternal sin—for they had said, “He has an unclean spirit.””
The Catechism of the Catholic Church teaches that, while no sin is absolutely “unforgivable”, some sins represent a deliberate refusal to repent and accept the infinite mercy of God; a person committing such a sin refuses God’s forgiveness, which can lead to self-condemnation to Hell. In other words, one damns oneself by final impenitence (refusal to repent), as taught by John Paul II:
Catholic Encyclopedia cites Matthew 12:22–32; Mark 3:22–30; Luke 12:10 (cf. 11:14–23) and defines “the unforgivable sin”—or sin against the Holy Ghost—as follows: ″… to sin against the Holy Ghost is to confound Him with the spirit of evil, it is to deny, from pure malice, the Divine character of works manifestly Divine.″ The article further states that “sin against the Son of Man” may be forgiven because it is committed against the human person of Christ, which veils the Divine with a “humble and lowly appearance,” and therefore such sin is excusable because it is committed through “man’s ignorance and misunderstanding.”
However, the Church further believes there is no offence, however serious, that cannot be taken away by Baptism, or absolved from in the Confessional—that no one, however wicked and guilty, may not confidently hope for forgiveness. The Catechism says that Christ desires “the gates of forgiveness should always be open to anyone who turns away from sin.” As did St Augustine, the Catholic Church today teaches that only dying unrepentant for one’s sins is the only unforgivable sin.