The unpardonable sin is committed when a person constantly rejects Jesus, knowing full well that He is the Messiah. When Jesus mentioned the unpardonable sin in Matthew 12:31–32, He was speaking to the Pharisees who rejected Him at every turn and were looking for ways to kill Him.
Those who have accepted Christ are in no danger of committing this sin, which is why the apostle Paul, who wrote two-thirds of the New Testament, never mentioned an unpardonable sin. Instead, he assures believers that all their sins have been forgiven because of Jesus' one sacrifice at the cross (Hebrews 10:12–14).