The 12th of the eighteen temple benedictions -
The blasphemous perfidious curse upon Christ and the Christians by the apostate Jews who thereby committed the unforgivable sin in the first century A.D.. . ."For the apostates let there be no hope. And let the arrogant government be speedily uprooted in our days. Let the noẓerim and the minim be destroyed in a moment. And let them be blotted out of the Book of Life and not be inscribed together with the righteous. Blessed art thou, O Lord, who humblest the arrogant" (Schechter).The noẓerim and the minim means exactly Christ and the Christians. This is the unforgivable sin, to curse Jesus Christ in this fashion, see:IT WAS GOD WHO HUMBLED THE JEWS INSTEAD. HE WILL DO SO AGAIN IN THE FUTURE.
[Mt.12:31] Therefore I say to you: Every sin and blasphemy shall be forgiven men, but the blasphemy of the Spirit shall not be forgiven.  And whosoever shall speak a word against the Son of man, it shall be forgiven him: but he that shall speak against the Holy Ghost, it shall not be forgiven him, neither in this world, nor in the world to come.
Mk.3; Amen I say to you, that all sins shall be forgiven unto the sons of men, and the blasphemies wherewith they shall blaspheme:  But he that shall blaspheme against the Holy Ghost, shall never have forgiveness, but shall be guilty of an everlasting sin.  Because they said: He [Jesus Christ] hath an unclean spirit.
1John 5: He that knoweth his brother to sin a sin which is not to death, let him ask, and life shall be given to him, who sinneth not to death. There is a sin unto death: for that I say not that any man ask.  All iniquity is sin. And there is a sin unto death. Little children, keep yourselves from idols. Amen.
v. 21 idols in Greek is eidooloon from eidoolatreia and means all idolatry and the very essence of it which is Satanic conjuring.
Together, these verses describe the sin of attacking the very source of grace who is the Holy Spirit and that sin will never be forgiven. This, however applies to those who know what they are doing. The tables are turned. Then it was the Jews who were guilty. Today, it is the false Christians - the Vaticanistas, who are guilty of the unforgivable sin in embracing Jewish Perfidy and Deicide or denying the teaching thereof and embracing Jewish apostasy. Today's Jews can be forgiven if they come sincerely to Christ and confess Him. The false Christians who engage in this are baptized and catechized and are therefore condemned in Hebrews 6th chapter utterly forevermore.