The greatest sin is blasphemy of the Holy Spirit because it is a sin that cannot be forgiven. Unbelief can be forgiven because many people have been unbelievers and then later became believers. If unbelief was unforgivable, then anyone who at any time did not believe in Jesus could not then be saved. Nevertheless, Jesus clearly tells us that there is a sin that will not be forgiven in this age or the age to come.
“Whoever speaks a word against the Son of Man, it shall be forgiven him; but whoever speaks against the Holy Spirit, it shall not be forgiven him, either in this age or in the age to come,” (Matthew 12:32).
In the context (Matthew 12:22-32), Blasphemy of the Holy Spirit the Pharisees were attributing the miraculous work of Jesus to the power of the evil one. Jesus had been casting out demons and the Pharisees said he was doing it by the power of Beelzebub. That is when Jesus gives his pronouncement about blasphemy of the Holy Spirit. Again, this is the only sin that is listed as unforgivable in the Scripture. To be clear, blasphemy of the Holy Spirit is saying that Jesus cast out demons by the power of Satan. This cannot be forgiven.
There are other serious sins, but they can be forgiven. For example, murder, rape, and theft can be forgiven. We know this because throughout history many murderers, rapists, and thieves have found redemption in Jesus, repented, and been forgiven. In the Bible, David had Uriah killed so he could have Uriah’s wife (2 Samuel 11:14–17). Yet, David was forgiven.
Paul the apostle was essentially murdering Christians yet he was forgiven. He was called by Jesus to be an apostle.
“But the Lord said to him, “Go, for he is a chosen instrument of Mine, to bear My name before the Gentiles and kings and the sons of Israel,” (Acts 9:15).
There are many great sins, but the greatest sin is Blasphemy of the Holy Spirit. Even denying Jesus in unbelief is not unforgivable since many unbelievers have become Christian.
Conclusion on the greatest sin
Since all kinds of sins are forgivable including rape, murder, line, and even unbelief, the only sin that is said to be unforgivable is blasphemy of the Holy Spirit. So, that is the greatest sin.
