The Scripture that gives us the closest look at this Unforgivable Sin is , for it describes it quite clearly: Mark 3:22-3022 And the scribes which came down from Jerusalem said, He [jesus] hath Beelzebub, and by the prince of the devils casteth he out devils.....(Cont.)28 Verily I say unto you, All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme:29 But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation:30 Because they said, He hath an unclean spirit. (KJV) The sin in the above Scripture was not that they insulted Jesus, but that they insulted the Holy Spirit by calling it an unclean spirit. Below we see that blasphemy against the Son of Man (remember the difference between Son of Man and Son of God) is forgivable, however, blasphemy against the Holy Spirit is not forgivable: Luke 12:1010 And whosoever shall speak a word against the Son of man, it shall be forgiven him: but unto him that blasphemeth against the Holy Ghost it shall not be forgiven. (KJV) Jesus wasn't saying that it was Ok to blaspheme Him, He was saying that it was forgivable to blaspheme Him while He was here in the flesh. Jesus, the Son of Man was born flesh, He was God but in a 'lower state', for the Scriptures tell us so: "But we see Jesus, who was made a little lower than the angels for the suffering of death...." (Heb 2:9). The Holy Spirit is the Spirit of the Lord God, to blaspheme that Holy Spirit was to blaspheme God Himself. To blaspheme Jesus Christ was to blaspheme God in the flesh. But that after He ascended, and the Holy Spirit was sent to 'replace' Him, such blasphemy against Him (Holy Spirit) was unforgivable. What is blasphemy? Bible Dictionaries will tell you that it is to insult God. The Scriptures tell us something different [edited for clarification: i.e., something additional]. Observe: John 10:3636 Say ye of him, whom the Father hath sanctified, and sent into the world, Thou blasphemest; because I said, I am the Son of God? (KJV)John 5:1818 Therefore the Jews sought the more to kill him, because he not only had broken the sabbath, but said also that God was his Father, making himself equal with God. (KJV)John 10:30-3330 I and my Father are one.31 Then the Jews took up stones again to stone him.32 Jesus answered them, Many good works have I shewed you from my Father; for which of those works do ye stone me?33 The Jews answered him, saying, For a good work we stone thee not; but for blasphemy; and because that thou, being a man, makest thyself God. (KJV)Phil 2:5-65 Let this mind be in you, which was also in Christ Jesus:6 Who, being in the form of God, thought it not robbery to be equal with God: (KJV) The Jews charged Jesus with blasphemy because He said that He was God, He said the He was equal with God. And had Jesus not been God in the flesh then the Jews would have been right to charge Him with blasphemy. But Jesus was God so He was not committing blasphemy by equaling Himself to God. This was the great error of the Jews, they did not accept Jesus for who He was, Immanuel — God with us, though their own Scriptures prophesied of Him.