First John 5.16, 17:
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. All wrongdoing is sin, but there is sin that does not lead to death.
I think it is a mistake here to assume that the sin or sins leading to death is some version of the “unforgivable sin.” I remember many years ago reading R. C. Spoul point out that the Chapter 21, paragraph 4, of the Westminster Confession is mistaken in using this passage as a rational for saying that it is forbidden to pray for those “for those of whom it may be known that they have sinned the sin unto death.” He made the obvious point that John does not say that such prayers are forbidden. But, if one thinks this refers to the “unforgivable sin,” then one can understand why readers would make that mistaken inference.My opinion is that John starts out making a point similar to Paul’s in Romans 14:
Who are you to pass judgment on the servant of another? It is before his own master that he stands or falls. And he will be upheld, for the Lord is able to make him stand.
Paul doesn’t want busybody lay “pastoring” to destroy the peace of the church and one of his tools of persuasion is to encourage Christians to be optimistic about one another. Likewise, in encouraging real love for the brethren, John encourages Christians to respond to the sins they see in other Christians not by active intervention but through private prayer. In such cases, John promises, God will give the person life. However, when you see something more serious (like a spouse having an extra-marital affair, say), John cannot promise that God will respond to a private prayer by giving life. That inability to be as positive about the outcome is why John says, “I do not say that one should pray for that.” The point is not that prayer is forbidden, but that he cannot make the same claim for such prayer that he made for lesser sins. Other action must also take place (i.e. Galatians 6.1ff).Thus, the various ways that Christians usually judge one another should not be used for interfering or for gossip, but for prayer. More serious sins that are ultimately a departure from the Faith, however, require rescue attempts.