The sin that leads to death is the one unpardonable sin.
This sin is rejecting the Holy SPirit.
The Reason this one sin leads to certain death is that the
unbeliever builds a wall were they refuse to trust in God's
forgiveness. If you don't have faith in and trust God
then you refuse His forgiveness.
So even this one sin could be forgiven, if only the unbeliever
had faith in God, which they don't.
So by refusing forgiveness, this leads them to Hell.
The one sin that does lead to death.
All other sins do not ensure death, becasue they can be forgiven.
But refusing God's forgiveness blocks salvation.
Don't you think it is kind of odd how John never mentions this kind of belief anywhere in his epistle? I see John mention the context of the "sin unto death" as willful sin that a person is justifying in some way and that they are not confessing with the genuine intent that they are going to forsake that sin (So as to be forgiven). How so?
Well, the "sin unto death" is the gnostic belief that justified willful sin. The gnostics were trying to seduce the brethren (1 John 2:26). So John was warning the brethren about the gnostic belief that held to the idea that sin was an illusion or sin did not exist for them (1 John 1:8) (1 John 1:10). John tells us to "sin not" (1 John 2:1). So sinning is not way of life for us. But if a believer happened to sin by some chance, they are to acknowledge the existence of sin by confessing it to Jesus (who is our advocate) so as to be forgiven (1 John 1:9) (1 John 2:1). No forgiveness means there is no salvation. Confessed sin (with the intent on forsaking it) is the "sin that is not unto death." Also, if we don't walk in the light, the blood of Jesus will not cleanse us of all sin (1 John 1:7) (Note: Walking (abiding) in the light is loving your brother - see 1 John 2:9-10; And loving your neighbor is the equivalent of keeping the Moral Law - see: Romans 13:8-10). For that person who says they know the Lord and does not keep His commandments is a liar and the truth is not in them (1 John 2:4). Proverbs 28:13 says he that confesses and forsakes sin shall have mercy.
"For this is the love of God, that we keep his commandments: and his commandments are not grievous." (1 John 5:3). Obviously this includes believing in Jesus (1 John 5:5) and being born again (1 John 5:4), which is the entrance and foundation of one's salvation. Immediately after John tells us about the
"sin uno death" (1 John 5:16), and the "
sin not unto death" in John 5:17, he says this:
18 "We know that whosoever is born of God sinneth not; but he that is begotten of God keepeth himself, and that wicked one toucheth him not.
19 And we know that we are of God, and the whole world lieth in wickedness."
(1 John 5:18-19).
1 John 5:18 is repeating 1 John 3:9. It is saying that whosever is in the born again state and abides in Jesus (1 John 3:6), does not sin (willfully) whereby they are seeking to justify sin (1 John 1:8). 1 John 5:18 says, He that is truly born again (or begotten of God) keeps himself by obeying God's commands and the wicked one touches him not. James 4:7 says, "Submit yourselves therefore to God. Resist the devil, and he will flee from you." 1 John 5:19 says we know we are of God (1 John 2:3), and the whole world lies in wickedness. But everyone who does evil hates the light (John 3:20). Men loved darkness rather than light, because their deeds were evil (John 3:19).
God is not a respecter of persons: But in every nation he that fears him, and works righteousness, is accepted with him (Acts of the Apostles 10:34-35)
Romans 2:11-12 says, “For there is no respect of persons with God. For as many as have sinned without law shall also perish without law: and as many as have sinned in the law shall be judged by the law;”.