This is not an easy verse to understand.  Although there is a similar statement in Matthew 12:31-32 and Mark 3:29-30, the context of those verses is of people who attributed the work of God to the power of Satan.  In Luke's account, the context is of the contrast between those who stand up for Jesus or who disown Him.  Yet, Luke records Jesus as saying that forgiveness is possible even for Peter who was going to disown Him (Luke 22:32) - and He prayed for His executioners to be forgiven when He was on the cross (Luke 23:34).

Even the disciples did not understand the significance of the crucifixion and resurrection of Jesus.  It took an encounter with the risen Christ and the Holy Spirit highlighting the Old Testament prophecies for them to honour Him as God’s Messiah (Luke 24:27-32).  All of the Early Church were converts – who once thought little or ill of Jesus, but changed their minds, including the Apostle Paul.   The same was true of Jesus’ own half-brothers (John 7:5).  

So, what is this unforgivable sin of blasphemy against the Holy Spirit?  Clearly it did not apply to those who heard Peter's Pentecost sermon because they were 'cut to the heart' (Acts 2:37), repented, believed, were baptised and received the Holy Spirit.  But for all who refused to repent, having heard the gospel and experienced the weight of the Holy Spirit's conviction, there was no other way to be saved and forgiven (Acts 4:12).  It is worth emphasising that sensitive believers who have sinned may be blackmailed by Satan with this verse into thinking that they cannot be forgiven.  As has been well said, if you are very worried that you may have committed the unforgivable sin, you have not!  Those who have do not care and their seared consciences seek no forgiveness.

The trainee apostles were being taught the frightening lesson that the Holy Spirit demands a positive response from all who hear the gospel.  To reject the message of salvation through Christ alone, is to reject the Holy Spirit who propels it into people's hearts (Acts 7:51).  Resisting His voice would leave no remedy for the rebellious soul (Hebrews 3:12-15).  The apostles, whose task was to take the gospel of Jesus to the world, needed to know that their message was not just interesting information for the religiously curious.  It demands a heaven or hell response.  And so, apostolic preaching was marked by both passion and persuasion.  The same challenge should stir the heart of every follower of Jesus, and especially those who are the mouthpieces of the Holy Spirit, teaching and preaching Christ.  We are responsible to keep on explaining the gospel (www.crosscheck.org.uk) with urgency.  All who hear are accountable for their own response.