Don’t all sins lead to death? In one sense, yes, of course, in light of God’s holiness and our guilt. But when John is talking to his children in 1 John 5 about sins leading to death and sins not leading to death, he has a specific sin in mind. This is made clear when he specifically says that we are not to pray for someone who commits that sin. He did not have to elaborate, as his audience knew what it was.
The writer to the Hebrews likewise warned against this ultimate sin, this sin which would require the very Messiah to die again for its forgiveness. It was apostasy—not the generic wandering off into the world for the love of its things, or simply become cold and dead spiritually. No, this was the act of offering sacrifice again in Judaism. This was how a person would ultimately deny Christ: by offering a goat or a dove or some other mere animal, one was saying, “All those things I confessed about Jesus being the final sacrifice, how the Father raised Him from the dead—it was all a lie, and this mere animal’s blood is more vicarious than that blood of Jesus of Nazareth.” It was truly the “sin unto death,” and it was so grave, so serious, that the Beloved Apostles says, “do not pray for such a person.”
But note that in the very same text where this dire sin is mentioned John likewise writes, “He who was born of God keeps him” (5:18). John recorded the words of Jesus in John 6, and 10, and 17. He knew that it is ultimately the Savior who saves, and He never loses one of His own.