If anyone sees his brother committing a sin not leading to death, he shall ask, and God will give him life—to those who commit sins that do not lead to death. There is sin that leads to death; I do not say that one should pray for that. 17 All wrongdoing is sin, but there is sin that does not lead to death.
18 We know that everyone who has been born of God does not keep on sinning, but he who was born of God protects him, and the evil one does not touch him. – 1 John 5:16-18
The man sat in my office and told me to my face, “I’m leaving my wife and family. I know I shouldn’t, but I’m going to do it. And you can’t stop me.” I didn’t know what to say. I should have recognized that as an opportunity to pray for this profoundly and tragically-deceived man. All too often, prayer is the last thing we do, or the smallest thing we do in regard to our brother having a difficult time. Prayer should be the first and the largest thing we do in the face of sin.
John says that we should pray for the one who is committing a sin “not leading to death.” The Reformation Study Bible speaks of this sin:
Some connect this sin with the unforgivable sin mentioned in Matt. 12:31, 32; Mark 3:28–30; Luke 12:8–10. More likely, John is referring to a stubborn refusal to accept the message of the gospel (1:10 note; John 8:24). See “The Unpardonable Sin” at Mark 3:29.
An on-line commentary speaks of the sin that leads to death:
There is a sin unto death; which is not only deserving of death, as every other sin is, but which certainly and inevitably issues in death in all that commit it, without exception; and that is the sin against the Holy Ghost, which is neither forgiven in this world nor in that to come, and therefore must be unto death; it is a sinning willfully, not in a practical, but doctrinal way, after a man has received the knowledge of the truth; it is a willful denial of the truth of the Gospel, particularly that peace, pardon, righteousness, eternal life, and salvation, are by Jesus Christ, contrary to the light of his mind,
We can try to understand what sins are “unto death” or those which are not “unto death.” But it seems better to me that we simply recognize that our first and largest move toward those who are obviously sinning is that of prayer. We can pray that the evil one does not indeed touch that brother or sister and that Satan would be given no foothold in his or her heart or life.