1 John 5:16 -- 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.
(Excerpt from BibleGateway, THE IVP NEW TESTAMENT SERIES -- SIN UNTO DEATH, SIN NOT UNTO DEATH (5:16-17), BibleGateway.com)
…But how does one "see" (5:16) another Christian committing a sin? Does this mean that it is a public or visible sin? Is the Elder referring only to kinds of sins that one can witness, such as actions, rather than thoughts? As is typical of the Johannine literature, "seeing" probably means "perceiving" or "understanding." If one has perceived—and John does not explain how one "perceives" this—that a fellow Christian is sinning, the proper response is to pray for that person. Presumably, that person has also repented and asked for forgiveness, for if the person who is sinning and is to be prayed for is indeed a brother or sister, a fellow Christian, then on John's view they would also be characterized by confession of sin and petition for pardon. THOSE WHO DO NOT ACKNOWLEDGE THEIR SINS TO GOD ARE NOT CHILDREN OF GOD.
And this gives us a clue why John prohibits prayers for those "whose sin is unto death" (5:16). This prohibition initially seems both hard-hearted and wrong-headed: surely these are precisely the people who most need prayer! The crucial question here is, For WHAT is one forbidden to ask? Verse 16 implies that one asks for life for the brother or sister who sins, just as Jesus asks for the life of Lazarus (Jn 11:41-43) and for forgiveness for the repentant sinner (1 Jn 2:1). Here, one asks for the confessing sinner to be held steadfast in eternal life (compare 5:18; Jn 17:11-15). Such a prayer can be made because this person (a) continues to be a faithful member of the community ("brother" or "sister"), which implies (b) that this person holds the Johannine confession of Christ and (c) acknowledges the sin to God (1:8—2:2). They have life, and prayer is made that they continue to receive life.
WHAT ONE MAY NOT ASK FOR WITH RESPECT TO THOSE WHOSE "SIN IS UNTO DEATH" IS THAT THEY BE GIVEN LIFE APART FROM THEIR REPENTANCE, CONFESSION AND RETURNING TO FOLLOWING CHRIST.