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 decision you have made.
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 it
was not accepted.
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 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. 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!"