"There is a sin that leads to death; I am not saying he should ask regarding that sin. 17All unrighteousness is sin, yet there is sin that does not lead to death." 1 John 5:16-17
I think the sin unto death is blasphemy of the Holy Spirit. That is, willful unbelief in the face of irrefutable evidence. The sin that, ultimately, once you are turned over to, you can not come back from.
That's fair.
An unconfessed sin leading to judgement of unbelief: them overthrown in the wilderness never confessed nor repented in their hearts, which God saw in their hearts and so were judged unbelievers: they were
trapped in the wilderness, and wanted to return to the slavery of Egypt where their sins were left behind.
So the 'sin of unbelief' would be sin unto unbelief.
So it would be any sin unconfessed and unrepented unto reprobation, where God gives up on the 'believer', because their faith is become dead by wicked works of the world, and become reprobate to every good work. I.e. it's not an 'isolated'
besetting sin.
We can sin seventy times seven and be forgiven seventy times seven
with confession.
However, we can also mock God by the carnal mindedness of
presuming confession and forgiveness. We can only confess from the heart
if God draws us to the cross. If He gives up on us by 'sinning away our day of grace': the space of time God gives to repent (Rev 2:21), then that is where there
remaineth no more sacrifice for sin, by putting Christ to an open shame in hypocrisy of faith.
Which is where Once Saved By Faith Only leads, especially when taught with all sins of future are
already forgiven.
Unconfessed and repented sin can also be false doctrine that leads to unbelief, or is unbelief in the beginning, such as Once Saved By Faith Only, which is a hard lie to be repented of, because it is so subtly tempting.
What carnal mind wouldn't want to believe they are going to heaven, no matter how they live, because they are forever 'saved' by
believing it only, and all future sins are as guaranteed forgiven as their salvation??
Sounds like the thinking of Adam and Eve in the garden: they will be as gods themselves with their own eternal life, independent of the commandments of the Lord!
It is a devilish recipe for disaster: We will surely sin and transgress, but it's already forgiven. So don't worry about it. Just feel bad for a little bit, and God will see our 'Godly sorrow', which is another lie. Godly sorrow
worketh repentance, which is not just mental sorriness, but
a work of repentance: Being sorry about being caught is the sorrow of the world, as was Esau.
