1Whoever believes that Jesus is the Christ has been born of God. Whoever loves the Father also loves the child who is born of him. 2By this we know that we love the children of God, when we love God and keep his commandments. 3For this is loving God, that we keep his commandments. His commandments are not grievous. 4For whatever is born of God overcomes the world. This is the victory that has overcome the world: your faith. 5Who is he who overcomes the world, but he who believes that Jesus is the Son of God?
6This is he who came by water and blood, Jesus Christ; not with the water only, but with the water and the blood. It is the Spirit who testifies, because the Spirit is the truth. 7For there are three who testify: 8the Spirit, the water, and the blood; and the three agree as one. 9If we receive the witness of men, the witness of God is greater; for this is God’s testimony which he has testified concerning his Son. 10He who believes in the Son of God has the testimony in himself. He who doesn’t believe God has made him a liar, because he has not believed in the testimony that God has given concerning his Son. 11The testimony is this: that God gave to us eternal life, and this life is in his Son. 12He who has the Son has the life. He who doesn’t have God’s Son doesn’t have the life.
13These things I have written to you who believe in the name of the Son of God, that you may know that you have eternal life, and that you may continue to believe in the name of the Son of God.
14This is the boldness which we have toward him, that if we ask anything according to his will, he listens to us. 15And if we know that he listens to us, whatever we ask, we know that we have the petitions which we have asked of him.
16If anyone sees his brother sinning a sin not leading to death, he shall ask, and God will give him life for those who sin not leading to death. There is sin leading to death. I don’t say that he should make a request concerning this. 17All unrighteousness is sin, and there is sin not leading to death.
18We know that whoever is born of God doesn’t sin, but he who was born of God keeps himself, and the evil one doesn’t touch him. 19We know that we are of God, and the whole world lies in the power of the evil one. 20We know that the Son of God has come and has given us an understanding, that we know him who is true; and we are in him who is true, in his Son Jesus Christ. This is the true God and eternal life.
21Little children, keep yourselves from idols.
This “if” may or may not be true. One may not see another Christian in the act of a sin. In this case, John describes the sin as “not leading to death.” He then affirms that some sins do lead to death. Thus, there are two kinds of sins: to death and not to death. John does not name examples, and so we cannot know.
The priestly function of believers is seen in what John says about praying for others. We can pray for God to spare the person, and God will “give him life.” We can intercede for one another, and God will “hear” our prayer and act on it in a positive way. However, there is an exception to this.
If God has determined a punish a Christian with physical death for a particular sin, there is nothing another believe can do for them. They are on their own. Examples of this in Scripture include Ananias and Sapphira (Acts 5) and the Corinthian who was committing incest with his stepmother (1 Cor 5).
Some interpreters see this sin leading to spiritual death. However, it is better to see it as physical death because the dying person is still a “brother.” Thus, it should be viewed as physical death. John’s point is that it is a waste of time to pray for that person because asking God to spare them is contrary to God’s will.
What is “sin unto death”? No one knows. God in His wisdom chose not to reveal it.