All their sins shall be forgiven unto the sons of men, etc. St. Mark adds the words (verse 30), "Because they said, [ ἔλεγον, 'they were saying,'] He hath an unclean spirit." This helps us much to the true meaning of this declaration.
Our Lord does not here speak of every sin against the Holy Spirit, but of blasphemy against the Holy Spirit. These words of St. Mark point to a sin of the tongue mere especially, although not excluding thoughts and deeds against the Holy Spirit.
Observe what these scribes and Pharisees did; they cavilled at works manifestly Divine—works wrought by God for the salvation of men, by which he confirmed his faith and truth. Now, when they spake against these, and knowingly and of malice ascribed them to the evil spirit, then they blasphemed against the Holy Ghost, dishonoring God by assigning his power to Satan.
What could be more hateful than this? What greater blasphemy could be imagined? And surely they must be guilty of this sin who ascribe the fruits and actions of the Holy Spirit to an impure and unholy source, and so strive to mar his work and to hinder his influence in the hearts of men.