1 John 5:16–17 has engendered much discussion about what John means by the “sin that leads to death.” This is one of those passages for which we have lost the key. Apparently, John’s readers knew what he was talking about, but we are left somewhat in the dark about it.
John does not explain what he means by the phrase “sin that leads to death.”
There are four major views as to what John means.
1. Some heinous sin like murder.
Yet there are many murderers whom God has forgiven. Paul comes immediately to mind. This interpretation is unlikely.
There are two variations here.
A. A person who was truly saved but commits apostasy, with subsequent loss of salvation.
The problem with this view is that genuine believers, in my understanding of Scripture, do not apostatize.
B. A person who is in the church, who is not a genuine believer, and who commits apostasy.
3. The unpardonable sin – blasphemy against the Holy Spirit.
The unpardonable sin is ultimate rejection of Jesus Christ. That’s the only sin you can commit in this life that God cannot and will not forgive. I think it is unlikely John is here talking about the unpardonable sin.
4. A sin committed by a Christian that results in God’s discipline by premature physical death.
I think this is the view that is most likely.
In the Bible, death can be either physical or eternal. John calls the one who commits the sin leading to death a “brother.” In almost every usage of the word “brother” in 1 John, John refers to believers. Apparently John is talking about the possibility that a Christian can sin in such a way that God may choose to take him prematurely out of this world by physical death.
Are there any examples of this in the New Testament? Yes! One example is found in Acts 5 with the account of Ananias and Sapphira. They sold a piece of property and pretended to give all of the proceeds to the church when in reality they kept back part of the money for themselves. They lied to God and to the church, and God struck each of them dead on the spot. Though some interpreters suggest Ananias and Sapphira were not genuine believers, there is no evidence in the text of Acts that such was the case.
Another example is found in 1 Corinthians 11:28–30. Some of the Corinthian believers were partaking of the Lord’s Table in an “unworthy manner.” As a result, some of them were “weak and sickly” and some had died prematurely as a result of God’s discipline.
You might say that John’s “sin that leads to death” is something of a “dishonorable discharge.” It has nothing to do with eternal salvation.
John says that God will give “life” to those who have not sinned a sin that leads to death. There are two possible ways to interpret “life” here.
It could be a reference to eternal life. It could also be a reference to physical life. Since the sin is one that leads to “death,” it is likely that the word “life” means the opposite of “death.” If the death referred to is spiritual, then so is the life. If the death referred to is physical, then so is the life.
Most likely, John is not talking about a specific sin (the Greek text probably should be translated “there is sin unto death”), but is referencing ongoing willful sin that remains unconfessed to the point that God takes drastic action.
John’s desire is that we not focus on the specifics of the sin that leads to death, but rather on the notion of “sin which does not lead to death.” That is what we are to pray about. God desires to forgive sins and he desires intercessors to pray for Christians when they sin. In the church, we need to be praying for one another. So, don’t just stand there, pray for somebody!
 Robert Yarbrough, 1–3 John, Baker Exegetical Commentary on the New Testament (Grand Rapids: Baker, 2008), 306–14, has a good discussion on this passage. In my opinion, from a preaching perspective, problem passages like this are best treated by giving all the major views and then presenting the one the preacher considers the most likely interpretation. Another option, though less satisfying homiletically, is to present all the views and leave it to the audience to pay their money and take their choice!
 Yarbrough proposes a variation of this view: “. . . the sin unto death will amount to specific manifestations of unregenerate conduct for which ‘blasphemy against the spirit’ serves as an umbrella rubric.” Yarbrough, 1–3 John, 308.