Matthew 12:31

Verse 31. All manner of sin and blasphemy] βλασφημια, injurious or impious speaking. [Anglo-Saxon], mocking and deriding speech, Anglo-Saxon. See Matthew 9:3.

But the blasphemy against the Holy Ghost] Even personal reproaches, revilings, persecutions against Christ, were remissible; but blasphemy, or impious speaking against the Holy Spirit was to have no forgiveness: i.e. when the person obstinately attributed those works to the devil, which he had the fullest evidence could be wrought only by the Spirit of God. That this, and nothing else, is the sin against the Holy Spirit, is evident from the connection in this place, and more particularly from Mark 3:28. "All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme; but he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation; BECAUSE they said, He hath an unclean spirit."

Here the matter is made clear beyond the smallest doubt - the unpardonable sin, as some term it, is neither less nor more than ascribing the miracles Christ wrought, by the power of God, to the spirit of the devil. Many sincere people have been grievously troubled with apprehensions that they had committed the unpardonable sin; but let it be observed that no man who believes the Divine mission of Jesus Christ, ever can commit this sin: therefore let no man's heart fail because of it, from henceforth and for ever, Amen. See below.