Jesus had just healed a person in the power of God, but the Pharisees accused Jesus of healing him in the power of Satan. In response, Jesus replied,
“So I tell you, every sin and blasphemy can be forgiven—except blasphemy against the Holy Spirit, which will never be forgiven. Anyone who speaks against the Son of Man can be forgiven, but anyone who speaks against the Holy Spirit will never be forgiven, either in this world or in the world to come.” (Matthew 12:31,32)
Blasphemy of the Holy Spirit is rejection of the Holy Spirit. In this particular context, the Pharisees were so hard hearted to the work of God that they could not discern between the work of God and the work of Satan. Anyone who accepts the work of God through Jesus on the cross is forgiven, they have the indwelling power of the Holy Spirit. Anyone who rejects the work and redemption of the Holy Spirit remains unforgiven and accountable to God for their own sin.
This particular passage primarily deals with a hard hearted person who continues to ignore the work of God, ignore the power of God, to the point that they can no longer recognize it. This was the main issue these particular Pharisees had. (note, not all pharisees were this way. Nicodemus and Paul of Tarsus are positive examples of Pharisees who responded to the Spirit’s work in their life.) Pharaoh in Egypt and King Saul are other chilling examples of leaders who ignored the work of God to their detriment and destruction.