Jamieson, Fausset and Brown on Matthew 12:31

31. Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men--The word "blasphemy" properly signifies "detraction," or "slander." In the New Testament it is applied, as it is here, to vituperation directed against God as well as against men; and in this sense it is to be understood as an aggravated form of sin. Well, says our Lord, all sin--whether in its ordinary or its more aggravated forms--shall find forgiveness with God. Accordingly, in Mark (Mr 3:28) the language is still stronger: "All sin shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme." There is no sin whatever, it seems, of which it may be said, "That is not a pardonable sin." This glorious assurance is not to be limited by what follows; but, on the contrary, what follows is to be explained by this.

but the blasphemy against the Holy Ghost shall not be forgiven unto men.