1It is actually reported that there is sexual immorality among you, and such sexual immorality as is not even named among the Gentiles, that one has his father’s wife. 2You are arrogant, and didn’t mourn instead, that he who had done this deed might be removed from among you. 3For I most certainly, as being absent in body but present in spirit, have already, as though I were present, judged him who has done this thing. 4In the name of our Lord Jesus Christ, when you are gathered together with my spirit with the power of our Lord Jesus Christ, 5you are to deliver such a one to Satan for the destruction of the flesh, that the spirit may be saved in the day of the Lord Jesus.
6Your boasting is not good. Don’t you know that a little yeast leavens the whole lump? 7Purge out the old yeast, that you may be a new lump, even as you are unleavened. For indeed Christ, our Passover, has been sacrificed in our place. 8Therefore let’s keep the feast, not with old yeast, neither with the yeast of malice and wickedness, but with the unleavened bread of sincerity and truth.
9I wrote to you in my letter to have no company with sexual sinners; 10yet not at all meaning with the sexual sinners of this world, or with the covetous and extortionists, or with idolaters, for then you would have to leave the world. 11But as it is, I wrote to you not to associate with anyone who is called a brother who is a sexual sinner, or covetous, or an idolater, or a slanderer, or a drunkard, or an extortionist. Don’t even eat with such a person. 12For what do I have to do with also judging those who are outside? Don’t you judge those who are within? 13But those who are outside, God judges. “Put away the wicked man from among yourselves.”
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 to punish a Christian with physical death for a particular sin, there is nothing another believer 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.