1 John 5:16, “If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. THERE IS A SIN UNTO DEATH: I DO NOT SAY THAT HE SHALL PRAY FOR IT.”
A contextual reading of the gospel of John from the very first chapter showed that in John’s day, there were those who denied the incarnation of Christ. They did not believe that Christ came in the flesh. Those who held to this view were the Gnostics. From that background, the book of John was written to counter that erroneous believe.
In context of the epistle of John, sin unto death is simply referring to the refusal to accept that Christ came in the flesh by someone who once professed the faith. The unpardonable sin is the willful denial of Christ with deliberate forethought. That is the sin unto death and it is a waste of time and energy to pray for the restoration of those that fall into that category because there is no repentance or mercy for them anymore. The door of mercy has been closed against them just like God did to Ephraim in Hosea 4:17.