Matthew 12:31

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 (Mar 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.

In this chapter, we have, I. Christ's clearing of the law of the fourth commandment concerning the sabbath-day, and vindicating it from some superstitious notions advanced by the Jewish teachers; showing that works of necessity and mercy are to be done on that day (Mat 12:1-13). II. The prudence, humility, and self-denial of our Lord Jesus in working his miracles (Mat 12:14-21). III. Christ's answer to the blasphemous cavils and calumnies of the scribes and Pharisees, who imputed his casting out devils to a compact with the devil (v. 22-37). IV. Christ's reply to a tempting demand of the scribes and Pharisees, challenging him to show them a sign from heaven (Mat 12:38-45). V. Christ's judgment about his kindred and relations (Mat 12:46-50).

12:31 In the face of the Pharisees’ accusations of Satanic influence, Jesus had just argued that he, the Messiah, cast out demons through the power of the Holy Spirit. In this context, blasphemy against the Holy Spirit can only mean attributing the ministry and exorcisms of Jesus to Satan’s power.