Eternal sins or unforgivable sins or unpardonable sins are part of Christian hamartiology, which is the Christian theology of sins. These are sins which will not be forgiven by God whereby salvation becomes impossible. One eternal or unforgivable sin is specified in several passages of the Synoptic Gospels: verse 29 in Mark 3 states that there is one sin considered eternal and that is "blasphemy against the Holy Spirit".
Several passages in the Bible are frequently interpreted as referring to the unforgivable sin:
- Mark 3:28-30: "Truly I tell you, all sins and blasphemes will be forgiven for the sons of men. But whoever blasphemes against the Holy Spirit will never be forgiven, but is guilty of an eternal sin. He said this because they [the Pharisees] were saying, ‘He has an evil spirit’."
- Matthew 12:30-32: "Whoever is not with me is against me, and whoever does not gather with me scatters. And so I tell you, people will be forgiven every sin and blasphemy. But the blasphemy against the Spirit will not be forgiven. Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come."
- Luke 12:8-10: "I tell you, whoever acknowledges me before men, the Son of Man will also acknowledge him before the angels of God. But he who disowns me before men will be disowned before the angels of God. And everyone who speaks a word against the Son of Man will be forgiven, but anyone who blasphemes against the Holy Spirit will not be forgiven."
- Hebrews 6:4-8: "It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, who have tasted the goodness of the word of God and the powers of the coming age and who have fallen away, to be brought back to repentance. To their loss they are crucifying the Son of God all over again and subjecting him to public disgrace. Land that drinks in the rain often falling on it and that produces a crop useful to those for whom it is farmed receives the blessing of God. But land that produces thorns and thistles is worthless and is in danger of being cursed. In the end it will be burned. Even though we speak like this, dear friends, we are convinced of better things in your case — the things that have to do with salvation."
- Hebrews 10:26-29: "If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left, but only a fearful expectation of judgment and of raging fire that will consume the enemies of God. Anyone who rejected the law of Moses died without mercy on the testimony of two or three witnesses. How much more severely do you think someone deserves to be punished who has trampled the Son of God underfoot, who has treated as an unholy thing the blood of the covenant that sanctified them, and who has insulted the Spirit of grace?"
According to the Catholic Catechism #1864, 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 hardness of heart can lead to final impenitence and eternal loss.
- despair: which consists in thinking that one's own malice is greater than Divine Goodness, as the Master of the Sentences teaches,
- presumption: if a man wants to obtain glory without merits or pardon without repentance
- resistance to the known truth,
- envy of a brother's spiritual good, i. e. of the increase of Divine grace in the world,
- impenitence, i.e., the specific purpose of not repenting a sin,
- obstinacy, whereby a man, clinging to his sin, becomes immune to the thought that the good searched in it is a very little one.
Thomas Aquinas explains that the unforgivability of blasphemy against the Holy Spirit means that it removes the entrance to these means of salvation—however, it cannot hinder God to take away this obstacle by way of a miracle.
According to one source however these are mortal sins against the Holy Spirit and not blasphemy against him though apart from presumption none of these are listed as mortal sins by the Catechism.
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 not being sorry for one's sins is the only unforgivable sin. Indeed in Dominum et vivificantem Pope John Paul II writes "According to such an exegesis, "blasphemy" does not properly consist in offending against the Holy Spirit in words; it consists rather in the refusal to accept the salvation which God offers to man through the Holy Spirit, working through the power of the Cross." and " If Jesus says that blasphemy against the Holy Spirit cannot be forgiven either in this life or in the next, it is because this "non-forgiveness" is linked, as to its cause, to "non-repentance," in other words to the radical refusal to be converted. This means the refusal to come to the sources of Redemption, which nevertheless remain "always" open in the economy of salvation in which the mission of the Holy Spirit is accomplished."
John Calvin wrote regarding the sin against the Holy Spirit:
"I say, therefore, that he sins against the Holy Spirit who, while so constrained by the power of divine truth that he cannot plead ignorance, yet deliberately resists, and that merely for the sake of resisting."
Similarly, Jacob Arminius defined it as "the rejection and refusing of Jesus Christ through determined malice and hatred against Christ". However, Arminius differed with Calvin in believing that the sin could be committed by believers, a conclusion he reached through his interpretation of Hebrews 6:4-6.
Some modern Protestant interpretations of the sin include the deliberate labeling of good as evil, as rejecting the conviction of the Holy Spirit, of publicly attributing the work of the Holy Spirit to Satan, and attributing the work of Jesus to Satan (under this interpretation, the sin could only have been committed in the first century AD). For example, The United Methodist Church, which was founded by John Wesley, upholds:
"that the penalty of eternal separation from God with no hope of return applies in scripture only in two cases—either, as in Hebrews 6 and 10, to persons who willfully, publically [sic] and explicitly reject Jesus as Savior after having confessed him, or, as in the gospels, to those who blaspheme against the Holy Spirit by declaring that the works of Jesus were the works of the Evil one.
According to Billy Graham not believing in God is the eternal sin 
Regardless of their interpretation, Protestant interpreters 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.
Members of The Church of Jesus Christ of Latter-day Saints, or Mormons, have a similar understanding of the eternal sin to mainstream Christianity. Joseph Smith, the founder of the Latter Day Saint movement, said in the King Follett discourse:
"All sins shall be forgiven, except the sin against the Holy Ghost; for Jesus will save all except the sons of perdition. What must a man do to commit the unpardonable sin? He must receive the Holy Ghost, have the heavens opened unto him, and know God, and then sin against him. After a man has sinned against the Holy Ghost, there is no repentance for him. He has got to say that the sun does not shine while he sees it; he has got to deny Jesus Christ when the heavens have been opened unto him, and to deny the plan of salvation with his eyes open to the truth of it; and from that time he begins to be an enemy."
- Mark 3:28-29, Matthew 12:31-32, Luke 12:10
- Cf. Catechism of the Catholic Church, para. 1864
- Summaria Theologiae. II/II 14 II
- Peter Lombard, Sent. ii. D43/2
- It must be mentioned in this place that the death-bed prayer of repentance is a meritorious act.
- Repentance itself need not be perfect repentance, i. e. as long as there is sorrow for the sin from love, or in the Sacrament of Penance fear, of God, and some will however weak to avoid grave sin and its nearest opportunities furtheron, there can be repentance: and it is better to repent from a sin and do it again, waiting maybe for a better time for another completer repentance, than not to repent from it at all until a perfect time in order to certainly never sin afterwards.
- S. th. II/II 14 III
- Roman Catechism I, 11, 5.
- Cf. Catechism of the Catholic Church, § 982; cf. Mt 18:21-22
- Catechism Of The Catholic Church Article 1864
- Unpardonable Sin by James Akin
- Dominum Et Vivificantem Article 46
- Calvin’s Institutes of the Christian Religion Book III Chapter III Section 22 (Translated by Henry Beveridge.)
- Combs, William W (2004). The Blasphemy Against the Holy Spirit Detroit Baptist Seminary Journal 9 (Fall 2004)
- Burton-Edwards, Taylor (2012). "Do United Methodists believe "once saved, always saved" or can we "lose our salvation"?". The United Methodist Church. Retrieved 14 June 2012. "Wesley notes that the penalty of eternal separation from God with no hope of return applies in scripture only in two cases—either, as in Hebrews 6 and 10, to persons who willfully, publically [sic] and explicitly reject Jesus as Savior after having confessed him, or, as in the gospels, to those who blaspheme against the Holy Spirit by declaring that the works of Jesus were the works of the Evil one.""
- The King Follett Sermon, Ensign, May 1971