If anyone sees his brother sinning a sin which does not lead to death, he will ask, and He will give him life for those who commit sin not leading to death. There is sin leading to death. I do not say that he should pray about that—1 John 5:16 NKJV
This passage is puzzling for many and, again, illustrates the importance of reading verses in context rather than isolating them from the whole which is a particular problem of the Christian habit of ‘proof-texting.’
Our first step, in the study of any of the epistles, should be to read the whole letter through. As we do that in 1 John, we come to the key clue, which clarifies the context,“They went out from us…”(2:19). See introduction to 1 John
The most plausible interpretation of this verse is that the “sin unto death” is like the apostasy of those who had been the brethren of these believers to whom John was writing, and who had followed new teaching that rejected the Incarnation.
“In that case, he [John] does not encourage prayer for the restoration of those who, like the teachers of 2. 18-23, had manifested the spirit of the Antichrist and shown where they properly belonged by quitting the fellowship [the Church] in which alone eternal life was to be found. With regard to such men John may have felt much as the writer of Hebrews did in another situation, that it was ‘impossible to renew them to repentance’; renunciation of the apostolic witness to Christ and His saving power was indeed a ‘sin unto death.’”—F. F. Bruce, Epistles…
“The person who consciously and deliberately chooses the way of death shall surely die. Sin that leads to death is deliberate refusal to believe in Jesus Christ, to follow God’s commands, and to love one’s brothers. It leads to death because it includes a deliberate refusal to believe in the One who alone can give life, Jesus Christ the Son of God. By contrast, sins that do not lead to death are those which are committed unwittingly and which do not involve rejection of God and his way of salvation.”—I. Howard Marshall NICNT
We can be distracted by such details and miss the important point: When we see a fellow Christian struggling with sin, it is urgent that we pray for him.
And we see the power of this as we read this verse in the context of Chapter 5: “. . . Now this is the confidence that we have in Him, that if we ask anything according to His will, He hears us. . . .”