Q&A: What does it mean to blaspheme the Holy Spirit?
First of all the word "blaspheme" means to utter obscenities or profanity, or to speak things that are irreverent and untrue about God.
The specific term in question, which is blasphemy against the Holy Spirit is recorded in Matthew and Mark.
Then they brought him a demon-possessed man who was blind and mute, and Jesus healed him, so that he could both talk and see. All the people were astonished and said, "Could this be the Son of David?" But when the Pharisees heard this, they said, "It is only by Beelzebub, the prince of demons, that this fellow drives out demons." Jesus knew their thoughts and said to them, "Every kingdom divided against itself will be ruined, and every city or household divided against itself will not stand. If Satan drives out Satan, he is divided against himself. How then can his kingdom stand? And if I drive out demons by Beelzebub, by whom do your people drive them out? So then, they will be your judges. But if I drive out demons by the Spirit of God, then the kingdom of God has come upon you. "Or again, how can anyone enter a strong man's house and carry off his possessions unless he first ties up the strong man? Then he can rob his house. "He who is not with me is against me, and he who does not gather with me scatters. And so I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven. Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come. Matthew 12:22-32 (NIV) This is also mentioned in Mark 3:22–30.
As you can see from the passage above, Jesus said that “blasphemy against the Holy Spirit” was literally speaking a word against the Holy Spirit. But specifically, He spoke of how the Pharisees claimed that Jesus was casting out demons through the agency of Satan. Jesus referred to this as speaking against the Holy Spirit, which He also called an unforgivable sin.
There are many who believe that this kind of blasphemy can no longer be done, primarily because the unique circumstances in which this was first spoken are no longer present. Those circumstances include the Pharisees’ public rejection of Jesus as Messiah in the face of all He had done among them. (Many have commented that never before in mankind's history had so much light and revelation been given to those who lived at that time.) To see and know as they did and still to attribute His works to Satan shows a willful blindness and hard-heartedness that is chilling. It appears these Pharisees revealed the extent of their wickedness by expressing this final rejection of Christ by those words of blasphemy.
I have spoken to many people over the years who have been plunged into a paralyzing fear over the belief that they have committed this unpardonable sin. I have told each one that their tenderness of heart and complete willingness to repent of any and all sin proves precisely the opposite. I don't believe the Pharisees had any concerns about having committed this or any other sin. That's what made their hearts so hard and led to their downfall. Those who are concerned about having committed this sin are among the most soft-hearted people I've ever had occasion to meet.
(Click here for a video teaching I did on this subject)