What is the unforgivable sin in the Bible?
Jesus called it blasphemy against the Holy Spirit (Matthew 12:31-32, Mark 3:28-29) - attributing the Spirit's work to Satan, as the Pharisees did when they credited Jesus's Spirit-empowered miracles to "Beelzebub." It is widely understood as a persistent, deliberate rejection of the Spirit's testimony about Christ, not a single sinful act.
In Matthew 12:22-32, Jesus heals a demon-possessed man, and the Pharisees respond by claiming he cast out demons "by Beelzebub the prince of the devils." Jesus answers that all manner of sin and blasphemy will be forgiven, "but the blasphemy against the [Holy] Ghost shall not be forgiven unto men." The parallel account in Mark 3:28-30 clarifies the context directly: "Because they said, He hath an unclean spirit" - that is, they attributed the visible, undeniable work of God's Spirit to the devil. Because of this context, most interpreters understand the unforgivable sin not as a single outburst or a sin committed in a moment of anger, but as a settled, willful rejection of the Holy Spirit's testimony about who Jesus is - a hardness of heart that refuses forgiveness by refusing the very means of forgiveness. Notably, the fact that a person is troubled about having committed it is itself often taken as evidence the Spirit is still working on their conscience.
Key Passages
Blasphemy against the Holy Ghost
Matthew 12:31-3231 Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men. 32 And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come.
Mark's account, with its context
Mark 3:28-3028 Verily I say unto you, All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme: 29 But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation: 30 Because they said, He hath an unclean spirit.
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