Matthew 12:31 says, “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.” This is always an interesting question to discuss because few people seem to really understand the nature of the “blasphemy against the Holy Spirit.” The context of Matthew 12 allows us to determine the meaning of this phrase. The incident began in v. 22 when Jesus healed a demon-possessed man. In v. 24, the Pharisees accused Jesus of casting out demons by Beelzebub, the ruler of demons. Jesus explained in vv. 25-30 how He could not have cast out demons by the power of Beelzebub.
In v. 31, Jesus made His claim that blasphemy against the Holy Spirit will not be forgiven. In the preceding verse, the Pharisees had already committed this crime. They had attributed the works of the Holy Spirit by whose power Jesus worked His miracles to the devil (v. 28). That sin was without excuse and would not be forgiven in their age or in the coming one (v. 32). These two ages referred to the Mosaical age and the gospel age.
In v. 32, Jesus explained why this sin was so severe. Speaking a word against the Son of Man because of unbelief or ignorance was pardonable, but trying to represent as Satanic the Spirit’s work in our redemption was unforgivable. It was unforgivable because if someone rejected Jesus, they still had the words of the Holy Spirit to convict them of sin. If they rejected the Spirit’s words, there was nothing else left to show them their error. Thus, the sin was unforgivable.