blasphemy against the Holy Spirit

I think the context in parallel passages is helpful (Matt. 12:22-37, Mark 3:20-35, Luke 11:14-28, 12:1-10). 

The Pharisees were staring straight at Jesus (the Messiah, the Incarnate Word) who performed miracles confirming His Identity by the power of the Holy Spirit.  They stared straight at him, and as the Holy Spirit was convicting them of sin, they obstinately and maliciously refused to believe that Jesus was the Christ.  Instead of repenting and believing, they blasphemed the Holy Spirit (that is, they attributed his power to the devil). 

The issue is much deeper than just a one-time sin, which is forgivable.  Any sin is forgivable.  But blasphemy against the Holy Spirit is the perpetual, white-knuckle, grit-your-teeth-until-you-die refusal to believe in Jesus and submit your life to His Lordship.  If you never humble yourself before the Holy Spirit, admit your sins, believe in the Lord Jesus, and commit your life to His Lordship, then your sins will stand against you on judgment day.

Jesus continues by pointing out that a tree is recognized by its fruit.  A good tree will produce good fruit that confirms the identity of the tree, and a bad tree will produce bad fruit that confirms the identity of the tree.  A person who rejects the Love and Mercy of Jesus Christ and the Holy Spirit's conviction will generally continue to become more and more selfish and destructive.  But a believer in Jesus Christ will, by the grace of God and the power of the Holy Spirit, grow in producing the Fruit of the Spirit.

I think the essence is captured in Luke 12:8-10, which parallels with John 3:16-21 and Romans 10:9-13.  This is the Gospel which we live and preach.  Do not harden your heart to the call of the Holy Spirit, but humble yourself and obey His gentle instruction - today and every day.