The sin unto death (which is deserving of death and will certainly bring eternal death) is denying the gospel of our Lord Jesus Christ. It is THAT SIN which kept most of Israel from the promised land–UNBELIEF (Heb. 3:19). The sin unto death is not a temporary fall, the transgression of a commandment, nor to be overtaken in a fault. The sin unto death is APOSTASY–to deny the precious person and work of our Lord Jesus. "He that hath not the Son, hath not life"–only death. We pray and receive forgiveness for all sin–The blood of our Lord Jesus Christ cleanseth us from all sin–but the man who refuses Christ and denies the merits of His blood and righteousness is condemned already. We are advised to not pray for such sin lest we be found contending with God's justice and trying to be more merciful than He!

Verse 18 of this same chapter encourages us with the promise that a true believer (born of God) will not deny Christ, but is kept and "the wicked one touches him not."