Many Christians have heard that there is an unpardonable sin and live in dread that something grave they have done before or after conversion might be that sin.
Their fears are unfounded. While there is an unforgivable sin, it is not one that a true believer in Jesus Christ can commit.
The one sin which God cannot forgive is mentioned in Mark 3:28-30 and Matthew 12:31-32. Jesus had been performing miracles, including driving demons out of people by the power of the Holy Spirit. Instead of recognizing the source of Jesus' power and accepting Him as God's Son, the religious leaders accused Him of being possessed by the devil and driving demons out in the power of the devil.
Jesus responded by saying, "I tell you the truth, all the sins and blasphemies of men will be forgiven them. But whoever blasphemes against the Holy Spirit will never be forgiven; he is guilty of an eternal sin."
The sin of the religious leaders, blasphemy against the Holy Spirit, was a refusal to accept the witness of the Holy Spirit to who Jesus was and what He had come to do, and then submit their lives to Him. Jesus said concerning the Holy Spirit, "When he comes, he will convict the world of guilt in regard to sin and righteousness and judgment" (John 16:8). They chose rather to reject the Spirit's witness to their sin and to Jesus, and accused Him of being demon possessed!
The point for us is that if we have received Jesus as our Savior and Lord, we have not blasphemed the Holy Spirit; we have accepted His witness. One study Bible explains it as follows: "To commit this sin one must consciously, persistently, deliberately, and maliciously reject the testimony of the Spirit to the deity and saving power of the Lord Jesus." If a person keeps doing that until death, there is no hope of forgiveness and eternal life in heaven.
Once again, the unpardonable sin is not some particularly grievous sin committed by a Christian before or after accepting Christ, nor is it thinking or saying something terrible about the Holy Spirit. Rather, it is deliberately resisting the Holy Spirit's witness and invitation to turn to Jesus until death ends all opportunity.
In order to experience God's peace, we must come to Him, trusting His promises. Isaiah 1:18 says, "Come now, let us reason together, says the Lord. Though your sins are like scarlet, they shall be as white as snow." In 1 John 1:9 we read: "If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness."
Jesus Himself assures us, "Whoever comes to me I will never drive away" (John 6:37). Our God is a compassionate and merciful God. He desires that no one should be lost, but that all should come to salvation through repentance and personal faith in Jesus as Savior and Lord (2 Peter 3:9; Acts 2:21).