What is the unpardonable sin?
Matthew 12:31 Therefore I say to you, any sin and blasphemy shall be forgiven men, but blasphemy against the Spirit shall not be forgiven.”
The unpardonable sin is the sin of rejection. The unpardonable sin is defined as blasphemy against the Holy Spirit, because when Christ was on earth, the religious leaders were attributing his miracles to demonic powers when in fact they were the product of the Holy Spirit working in the life of Christ (Matt. 12:12-32). This demonstrated a heart that had become so hard that it precluded repentance because Christ’s words and works were a clear evidence of the ministry of the Holy Spirit in Him as the Anointed One, the Messiah of the Old Testament. To reject Christ and claim His works were from Satan was a clear evidence of both unbelief and an unrepentant heart. That’s why it was considered unpardonable.
The one sin that is unforgivable is unbelief or rejection of Christ. Note in the passage quoted below that the convicting work of the Spirit is related to giving demonstrable proof that it is the sin of rejection that is the issue.
8 And He (the HS), when He comes, will convict the world concerning sin, and righteousness, and judgment; 9 concerning sin, because they do not believe in Me; 10 and concerning righteousness, because I go to the Father, and you no longer behold Me; 11 and concerning judgment, because the ruler of this world has been judged (John 16:8-11).