Could you explain what John means in 1John 5:16 about the "sin leading to death?"
In 1 John 5 we read:
1John 5:12 He who has the Son has the life; he who does not have the Son of God does not have the life.
1John 5:13 These things I have written to you who believe in the name of the Son of God, so that you may know that you have eternal life.
1John 5:14 This is the confidence which we have before Him, that, if we ask anything according to His will, He hears us.
1John 5:15 And if we know that He hears us in whatever we ask, we know that we have the requests which we have asked from Him.
1John 5:16 If anyone sees his brother committing a sin not leading to death, he shall ask and God will for him give life to those who commit sin not leading to death. There is a sin leading to death; I do not say that he should make request for this.
1John 5:17 All unrighteousness is sin, and there is a sin not leading to death.
At the beginning of this passage, John defines what he means by "death" and" life." Life is having the Son, that is to be a believing Christian. Conversely, not having life (i.e., death) is not having the Son, that is being an unbeliever. The Bible commonly uses the dichotomy of life and death to represent belief and unbelief. John repeats this association several times in this letter, including again in v.13 when he says that the church's belief in Christ has given them eternal life.
Then he teaches that believers have the confidence to approach the Lord with our petitions, and the Lord will hear us. And if the Lord hears us, John says we know we "have the requests which we have asked from Him." This is an awkward phrase in the English reflecting a difficult Greek meaning. John is saying that if the Lord grants our requests, it means that our requests were according to the Lord's desires.
Furthermore, when we intercede for the needs of a brother or sister who is sinning, we can have confidence that the Lord's assurance of eternal life for that believer is not in jeopardy on account of the person's sin. In other words, a believer's sin will be forgiven.
On the other hand, there is a sin that leads to eternal death, which is the sin of unbelief, and this sin cannot be forgiven through our intercession. We have no hope that the Lord will grant our petition that someone be forgiven of their sin of unbelief. The Lord cannot grant such a request.
Therefore, concerning a person committing the sin of unbelief, John says the Christian ought not to pray for forgiveness for that person since we know it cannot be granted. We would not be praying in the Lord's will in such a case. Instead, we should pray for the individual to believe so as to be saved and forgiven.
Finally, John says that though all unrighteousness is sin, there is a difference between the sin of unbelief and the sins committed by believers. Sin in the life of a believer does not endanger our eternal life, while the sin of unbelief prevents forgiveness.
