The reference to this “sin unto death” (which is singular in contrast to sins not leading to death, which is plural) is found in 1 John 5. And though John never explicitly tells us what this sin is, my opinion is that this sin is the sin of final rejection/apostasy in which it is clearly manifest you are not a “brother” after all, someone without the Son and without “life” (1 John 5:12). So all that is left for that one is death and its ultimate penalty of separation from God. (The context of the passage and the flow of the book are the two primary things that push me in that direction pretty heavily. You can hear me explain this in this brief clip from my message on this passage.)
Some wonder, however, if this could actually refer to any sin that is left unreported of, as opposed to a specific sin? In other words, “wouldn’t it be safe to say that sin that leads to death is unrepentant sin?”
I don’t think so. In fact, John actually says in 5:17 that “there is sin that does not lead to death.” This is why I think the “sin unto death” is the specific sin of final rejection/unbelief, not just any sin which may be left over and of which we haven’t repented. If we go down the “any sin” road, we end up at a place that says if you die with unrepented sin, you end up separated from God. Yet, the Bible says that unbelief/rejection of Jesus is what separates us from God eternally and keeps us under his wrath (John 3:36). Most specifically, it’s the sin of refusing the Son that leads to death in its most ultimate and eternal sense.
Don’t misunderstand me – I’m not making light of sins, or minimizing their effect on us. But I don’t think the “sin unto death” is any unrepented sin, but rather refusing to repent of the sin of unbelief in Jesus. It is that rejection that leaves us with — and leads us to — nothing but a tragic death march.