Is there a sin that can never be forgiven?

Both statements are true together, because Paul’s promise is to “all that believe”, and Jesus’ warning is about people who will not believe. The occasion shows what the sin was. Jesus had freed a blind and dumb man from a demon, and the Pharisees and scribes, unable to deny it, said he did it by Beelzebub. Mark gives the reason for the warning: “Because they said, He hath an unclean spirit” (3:30). They were calling the Spirit’s plain work of mercy the work of the devil, with their eyes open.

It is not every insult to God. Jesus begins with a sweeping promise, “All manner of sin and blasphemy shall be forgiven unto men” (Matthew 12:31), and even “a word against the Son of man” can be forgiven (12:32). Paul had been “a blasphemer, and a persecutor”, and was forgiven because he acted “ignorantly in unbelief” (1 Timothy 1:13). The sin cannot be forgiven, not because Christ’s death is not enough, but because it is a settled refusal of the Spirit through whom God brings people to repentance and faith; whoever persists in it never seeks forgiveness. So Acts 13:39 and Matthew 12:31 agree: forgiveness is offered for every sin to all who believe, and only the one who finally refuses it goes without it. As Matthew Henry observed, those who fear they have committed this sin show by that very fear that they have not.

Only in Jesus’ ministry

Some, such as Got Questions, hold that this sin required seeing Jesus’ miracles in person and calling them the work of Satan, so it cannot be committed in the same way today; the nearest thing now is persistent unbelief to the end.

Final impenitence

Augustine, and much Catholic teaching after him, identified the sin with the hardness of heart that refuses repentance to the end of life.

Whether this sin can be committed today, and exactly what it consists of, are debated among Christians.