Question: What is the unpardonable sin? My brother is convinced he has committed it...has horrible nightmares and has tried to end his life over this.
Answer: First, the term "unpardonable sin" does not appear in the Bible. There is no "unpardonable" or "unforgivable" sin, except in this sense. If you die without trusting Christ as your Savior, your destiny is Hell and it is too late to change the destination you have chosen. Your sins were all paid for on Calvary when Christ died for the sin of the world. (John 3:16). If you die without believing that Christ did this for you; i.e. accepting the forgiveness He provided, then you end up in Hell. (John 3:18, John 3:36). The forgiveness was offered; but you refused it.
Probably someone told your brother that he had "blasphemed the Holy Ghost" and showed him Matthew 12:31, 32.
"Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men; but the blasphemy against the Holy Ghost shall not be forgiven unto men. And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world ("age"), neither in the world (or "age") to come."
(The companion passage is Mark 3:28-30.)
"This age" is the Dispensation of Law: the time before the crucifixion when Israel (nationally, as a group) rejected Christ as their Messiah. They were not a legal nation, having a land and a king; but, were governed by the Sanhedrin under the permission of Rome. "The age to come" is the Dispensation of Grace, or the Church Age, where "There is neither Jew nor Greek...for ye are all one in Christ Jesus." (Galatians 3:28).
What was the "blasphemy against the Holy Ghost" or the "speaking against the Holy Ghost" that the Nation of Israel was not to be forgiven of in "this age" or "the age to come?" It was rejecting the conviction of the Holy Spirit that Jesus Christ was their Messiah. John 1:11 says "He came unto his own, and his own received him not." This is proven by the fact that Christ addressed these verses to the "Pharisees" (Matthew 12:24) and "...the scribes which came down from Jerusalem." (Mark 3:22). These were the "Seminary Professors" who told the Nation of Israel what to believe and refused to accept Christ as the promised Messiah.
This was a dispensational sin committed by the Nation of Israel. The sin was not forgiven in the Age of Law or the "age to come," the Church Age; but will be forgiven. When? When Israel is restored as a Nation with a land and a king, the Lord Jesus Christ, in the Millennium.
"For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in. (25) AND SO ALL ISRAEL SHALL BE SAVED: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob." - Romans 11:25,26
Tell your brother, "If he has accepted Jesus Christ as his personal Savior, he cannot be guilty of the unpardonable sin of unbelief." Why? Because he believes!"
"Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life." - John 5:24