Why Jesus gave this warning
Jesus' warning came when scribes from Jerusalem accused him of casting out demons by the power of Beelzebub, another name associated with Satan. Jesus answered that Satan would not work against his own kingdom. A kingdom divided against itself cannot last (Mark 3:22-26).
Mark explains the immediate reason for the warning: these leaders were saying that Jesus had an unclean spirit (Mark 3:30). Matthew calls their accusation blasphemy against the Spirit. Blasphemy means speaking about God with contempt or slander. They were not simply confused or asking honest questions. They were labelling the Spirit's work in and through Jesus as evil (Matthew 12:31-32).
Verily I say unto you, All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme: But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation:
What makes it unforgivable
This does not mean God's mercy runs out after a certain number of failures. Jesus came to call sinners to repentance, and he described his blood as given for the forgiveness of sins (Luke 5:32; Matthew 26:28). Forgiveness is open to people who turn to him.
The problem is a settled refusal of the very One who brings a person to repentance. The Holy Spirit points people to Jesus and exposes what is true. When someone persistently and knowingly rejects that light, calling it darkness, they refuse the forgiveness they need. The warning describes a hardened posture, not one careless phrase or terrible day.
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. 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.
What it is not
A disturbing thought is not the unforgivable sin. Neither is doubt, anger, a past season of rebellion, or words spoken while distressed. Jesus addressed people making a deliberate accusation about his work, not a struggling person who wants to believe or return to God.
Peter denied even knowing Jesus when he was under pressure. He later wept bitterly, and after his resurrection Jesus restored Peter and entrusted him with caring for others (Luke 22:61-62; John 21:15-17). Peter's denial was serious. Yet it was met with grace when he turned back.
If this worries you
If you are afraid you have committed this sin and you want God's forgiveness, don't let fear drive you away from him. Bring that fear to Jesus. The leaders in this passage were not seeking mercy. They were resisting him.
Speak honestly to God about what troubles you. Confess what you know is wrong, ask for help to trust him, and read the Gospel accounts slowly. God promises forgiveness and cleansing to those who confess their sins (1 John 1:9). This warning is serious, but its purpose is to call us towards truth, repentance, and life.
If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.