Question: "What is the sin that does not lead to death in 1 John 5:16–17?"
Answer: A sin that does not lead to death (and a sin that does lead to death) is alluded to in 1 John 5:16–17: “If you see any brother or sister commit a sin that does not lead to death, you should pray and God will give them life. I refer to those whose sin does not lead to death. There is a sin that leads to death. I am not saying that you should pray about that. All wrongdoing is sin, and there is sin that does not lead to death.”
John wrote his gospel so that people would believe in Jesus and have life in His name (John 20:30–31). He wrote his first epistle, in part, so that those who believe in Jesus would know that they have eternal life (1 John 5:13). John wants believers to have confidence in their position in Christ and mentions in that context that there are a couple types of sin. One type of sin does not lead to death, and another does.
We find an example of a sin that leads to death in Acts 5. There, Ananias and Sapphira lie to the Holy Spirit (Acts 5:3), and they die as a result. Paul mentions another example of a sin that leads to death in 1 Corinthians 11:30. Some who were abusing the Lord’s Supper had become sick and had even died. These are the only two clear instances in the New Testament of believers who committed sins unto death. John mentions a sin unto death in his first epistle, but he does not offer any specifics regarding what the sin is. He is speaking of categories of sin, not pointing out specific sins.
The context is John’s teaching on prayer. John explains that we can have confidence that, in anything we ask of God according to His will, He hears us (1 John 5:14). To ask something according to His will is to ask for something that God has communicated to us that He desires. When we ask for something that He wants, then we are asking for something that we should be asking Him for. John goes further, explaining that, when God hears these kinds of requests, we have what we have asked for (1 John 5:15). Believers are to pray without ceasing (1 Thessalonians 5:17), and, as Jesus said in John 14:12–14, the Father is glorified in the Son when His disciples pray in His name. To pray in Jesus’ name is to pray according to His will or pray as if Jesus Himself were asking the Father. But we discover in 1 John 5 there are different types of sin, and that impacts what we should and should not be asking for. The church must recognize the seriousness of sin, avoid continuing in sin, and pray as God has designed.
It is important to note the qualifier that John (and Jesus) places on prayer. God does not promise to do whatever we ask without condition. He affirms that He will grant what is asked according to His will, in the name of Jesus. Jesus was not referring to a magic formula or mantra or suggesting we should add the phrase in Jesus’ name to our prayers in order to ensure they are granted. We should be asking God for what He has revealed He wants for us. But there are some things that John explains we ought not to ask for. He provides an example: if a brother commits a sin not unto (or does not lead to) death, then the one observing the sin should ask for God’s mercy for the offender so that death does not result (1 John 5:16). On the other hand, there is a sin that does lead to death, and John does not suggest one should pray on behalf of the one committing that type of sin (1 John 5:16). John distinguishes between these two kinds of sin even as he acknowledges that all sin is unrighteousness (1 John 5:17).
The distinction John makes between the sin that does not lead to death and the sin that does lead to death illustrates that there are some requests not in accordance with God’s will and, thus, some requests the believer should not expect to be granted. This principle is simple and straightforward. If we’re praying for things outside of God’s will, then we should not expect an answer.
The challenge for interpreters is that nowhere in the context does John detail what sins he is talking about. He speaks of those sins as broad categories. If he had specific sins in mind, no doubt his original readers would understand to what he referred. Because of the ambiguity, it is probably best to simply acknowledge the principle being taught about prayer rather than try to identify what John doesn’t provide in the context—the specific nature of the sin that does not lead to death and the sin that does.