1 John 5:16-17: "If any one sees his brother committing what is not a mortal sin, he will ask, and God will give him life for those whose sin is not mortal. There is sin which is mortal; I do not say that one is to pray for that. All wrongdoing is sin, but there is a sin which is not mortal."
Luke 12:47-48: "And that servant who knew his master's will, but did not make ready or act according to his will, shall receive a severe beating. But he who did not know, and did what deserved a beating, shall receive a light beating. Every one to whom much is given, of him will much be required; and of him to whom men commit much they will demand the more."
"The first thing to note is the context. This major topic from 5:13-18 is prayer. We are given in verses 13-15 that God hears and responds to our prayers. The key word is "anything." Then John remembers there is an exception: praying for a disobedient, sinning brother or sister in Christ. What to do? How do we pray for that one? Here is the sequence we must keep in mind for such a one as we pray. First of all, the Apostle John tells us that there is a sin not leading to death (physical). In verse 16, he tells us that it is possible for Christians to fall into this sin not leading to death. [See also 1 John 2:1,2--the ideal is to "sin not." But if anyone sins (and we will), we have an Advocate, a defense attorney.]
When Christians observe disobedience in brothers and sisters, they are to pray for him/her (16b); as a result of these prayers, God may choose to preserve, prolong, extend the person's physical life (not eternal life, since that life is determined by one's personal faith decision). This intercession is effective only in the case of sin not leading to death (16c): that is, the person has not reached the end limits of God's patience and grace (His "last straw"). See also v. 17 where John says, "All unrighteousness is sin, but there is a sin which is not unto (physical) death."
Secondly, there is a sin which results in physical death--the sin unto death (v. 16d): This is the death of a believer characterized by persistent, willful sinning in which "the flesh is destroyed [physical death--1 Cor. 5:1-5] so that the spirit might be saved." John tells us that this is a sin not to be prayed for, because God's immutable law concerning this final, "last straw" disobedience is involved and will be unaltered by intercessory prayer (16e), and frankly, we do not know another's heart condition before the Lord. We are not encouraged to speculate about the cause of any believer's untimely death. In our prayer life, we can continue to intercede for a wayward brother or sister, but we are not to draw any conclusions about what may, should, or has happened in regard to a believer's death.
Thirdly, when some Christian we know dies, we might be inclined to ask the question of ourselves, "Was this the sin unto death or not?" John is telling us in this passage not to speculate, because we just don't know.
All through this Epistle (1 John) the Apostle has been addressing sin in the life of the believer--yours and every Christian you know. It is fitting that John portrays the remedy of habitual sin on the part of a believer in the context of the new birth."