Many believers have often questioned the meaning behind what is referenced in parallel in Mark 3:29, Luke 12:10, and Matthew 12:32 which state that someone can commit an unforgivable sin by blaspheming the Holy Spirit.
There is a popular movement amongst an atheist group called the Rational Response Squad who have issued the “blasphemy challenge” to encourage people to publicly state that they deny the Holy Spirit. They encourage this because they think that this means that a person can never be forgiven by Christ and thus not saved. In my opinion, it seems like they are expending a lot of energy speaking against someone who they believe does not exist!
Anyway, upon a careful examination of this text we find the real meaning behind what Jesus was saying. At this particular moment, Jesus is talking to a group of men called Pharisees. They were one of the strictest sects of Judaism. He had been showing them who he was through miracles, wise teaching, and fulfillment of prophecy for a good amount of time and yet they still did not believe. In verse 32 of Matthew 12, Jesus lays it out for them in a very simple way. The Holy Spirit had clearly been active in Jesus’ ministry and the Pharisees were claiming that Jesus did this through the power of Satan. So even though the truth of Jesus’ claim to be the Messiah was revealed to them, they still would not believe. So they denied Jesus as their Lord and Savior and thus this is an unforgivable sin because in their unbelief they would spend eternity in Hell. The Bible Knowledge Commentary points out that while many of these particular Pharisees would never become believers, Jesus’ brothers initially rejected Jesus as Messiah (John 7:5), but would later come to faith (Acts 1:14). So while at some point in our lives, in our ignorance of God we may speak against Jesus, we may still be moved to believe by the Holy Spirit, but if the truth is revealed to us and we do not respond to the call to believe by the Holy Spirit then when we die we will not have a chance to respond to the invitation to follow Christ. (See Hebrews 10:26-27). This is the unforgivable sin...An unrepentant heart which knows not Jesus Christ as it's Lord and Savior.
1 John F. Walvoord, Roy B. Zuck and Dallas Theological Seminary, The Bible Knowledge Commentary : An Exposition of the Scriptures (Wheaton, IL: Victor Books, 1983-c1985), 2:237.