During a time when Jesus is perhaps somewhere in Galilee, he is surrounded by crowds and a demon-possessed man, who is blind and mute, is brought before him for a healing.
Jesus heals the man who is then able to see and speak.
The miracle causes people in the crowd to be amazed and they ask themselves if Jesus is the Messiah, the promised "son of David," which is a reference to Old Testament prophecies that the Messiah would be a descendant of King David, who was ancient Israel's greatest king.
The religious leaders who opposed Jesus throughout his public ministry, however, do not share the crowd's favorable reaction. They accuse Jesus of getting his power from Satan, who also was known as Beelzebul.
Jesus replies that the accusation is false, and that it is absurd to think that Satan would grant the power to overpower himself. Jesus also speaks of a sin that cannot be overcome -- the sin of refusing and denying the Holy Spirit. Jesus indicates in Matthew 12:28 that it is by the Holy Spirit that he drives out demons.
This sin is referred to as the "unforgivable sin," the "unpardonable sin" and the "eternal sin."
Christian theologians have offered a variety of interpretations for the unpardonable sin. Some say that it involves a perpetual and permanent refusal to repent:
"Ultimately, once a person has either hardened his or her heart to an irretrievable point in this life or has died without repenting, the chance for forgiveness has passed. It is only what one does in this life that matters for eternity. Thus the only true ‘unpardonable sin’ is when a person consciously, willfully, rejects the operation of the Spirit bearing witness to the reality of Jesus as the Savior. The person who does not receive this work of the Spirit cannot come to Jesus and therefore cannot receive forgiveness." - Michael J. Wilkins, NIV Application Commentary.
This definition by Wilkins shares common ground with many other answers, that the unpardonable sin involves a lifelong refusal to accept Jesus and his gift of salvation:
"Blasphemy against the Spirit is unforgivable because it means denying Jesus Christ is who He says He is and denying His gift of salvation. This is the unpardonable sin because you are literally rejecting the only One who can save you. God isn't going to force you to accept Jesus as your Savior. However, if you refuse to trust Christ, then you will face the consequences of your decision, which are judgment and condemnation (John 3:18). God will never reject anyone who is truly repentant. And if you’ve accepted Jesus as your Savior, then you can never commit the unpardonable sin." - Charles F. Stanley Life Principles Bible, NASB.
This event is described in the Gospel of Matthew:
25 Knowing their thoughts, Jesus said to them, “Every kingdom divided against itself is brought to desolation, and every city or house divided against itself will not stand. 26 If Satan casts out Satan, he is divided against himself. How then will his kingdom stand? 27 If I by Beelzebul cast out demons, by whom do your children cast them out? Therefore they will be your judges. 28 But if I by the Spirit of God cast out demons, then God’s Kingdom has come upon you. 29 Or how can one enter into the house of the strong man and plunder his goods, unless he first bind the strong man? Then he will plunder his house.
30 “He who is not with me is against me, and he who doesn’t gather with me, scatters. 31 Therefore I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven men. 32 Whoever speaks a word against the Son of Man, it will be forgiven him; but whoever speaks against the Holy Spirit, it will not be forgiven him, either in this age, or in that which is to come.
33 “Either make the tree good and its fruit good, or make the tree corrupt and its fruit corrupt; for the tree is known by its fruit. 34 You offspring of vipers, how can you, being evil, speak good things? For out of the abundance of the heart, the mouth speaks. 35 The good man out of his good treasure brings out good things, and the evil man out of his evil treasure brings out evil things. 36 I tell you that every idle word that men speak, they will give account of it in the day of judgment. 37 For by your words you will be justified, and by your words you will be condemned.” (Matthew 12:22-37, WEB)
Read more in the book Jesus the Miracle Worker: The 50 miracles of Jesus explained in chronological order (The Jesus Series) -- now available at amazon.