When scholars claimed that Jesus was working through the power of Satan Jesus warned them of the blaspheming of the Holy Spirit. The blaspheming of the Holy Spirit is conscious opposition of the work of the Holy Spirit. There are similar exhortations elsewhere in the Bible too. The letter to the Hebrews speaks of conscious sin, which leads to a permanent separation from God. Sin in this connection does not speak of any sin but of a conscious denial of faith.

The words of Jesus and the letter to the Hebrews is not to be understood as a condemnation to anyone, but as a warning to those who have not yet committed this irreversible sin. It is important to remember that the door on God’s side is open as long as the good news of Jesus Christ are being proclaimed. The problem for one that is guilty of blaspheming the Holy Spirit is not whether God would forgive him or not, but his unwillingness to return to God - the heart has been hardened. Therefore he who is afraid of having committed the irreversible sin, can be sure that he has not done it yet, because repentance is a sign of a heart that still listens to God’s calling.  

And whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.
(Matthew 12:32, ESV)