Question: "What is the unpardonable sin / unforgivable sin?"
Answer: The case of the "unpardonable sin/unforgivable sin" or "blasphemy of the Holy Spirit" is mentioned in Mark 3:22-30 and Matthew 12:22-32. The term "blasphemy" may be generally defined as "defiant irreverence." We would apply the term to such sins as cursing God or willfully degrading things relating to Him. It is also attributing some evil to God, or denying Him some good that we should attribute to Him. This case of blasphemy, however, is a specific one called "the blasphemy against the Holy Spirit" in Matthew 12:31. In this passage, the Pharisees, having witnessed irrefutable proof that Jesus was working miracles in the power of the Holy Spirit, claimed instead that He was possessed by the demon Beelzebub (Matthew 12:24). In Mark 3:30, Jesus is very specific about what exactly they did to commit "the blasphemy against the Holy Spirit."
This blasphemy then has to do with accusing Jesus Christ (in person, on earth) of being demon-possessed. There are other ways to blaspheme the Holy Spirit (such as lying to Him, as in the case of Ananias and Sapphira in Acts 5:1-10), but the accusation against Jesus was the blasphemy that was unpardonable. This specific unpardonable sin against the Holy Spirit cannot be duplicated today.
The only unpardonable sin today is that of continued unbelief. There is no pardon for a person who dies in unbelief. John 3:16 tells us, "For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life." The only condition in which someone would have no forgiveness is if he/she is not among the "whoever" that believes in Him. Jesus said, "I am the way and the truth and the life. No one comes to the Father except through me" (John 14:6). To reject the only means of salvation is to condemn oneself to an eternity in hell because to reject the only pardon is, obviously, unpardonable.
Many people fear they have committed some sin that God cannot or will not forgive, and they feel there is no hope for them, no matter what they do. Satan would like nothing better than to keep us laboring under this misconception. The truth is that if a person has this fear, he/she needs only to come before God, confess that sin, repent of it, and accept God's promise of forgiveness. "If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness" (1 John 1:9). This verse assures us that God is ready to forgive any sin—no matter how heinous—if we come to Him in repentance. If you are suffering under a load of guilt today, God is waiting with His arms open in love and compassion for you to come to Him. He will never disappoint or fail to pardon those who do.