Sin is a transgression of the law (I John 3:4). If sin is left unchecked in the soul of man it will lead to spiritual death, that is separation from God for all eternity (James 1:15). Eternal condemnation is the payment or wages of sin (Rom. 6:24). Jesus shed His blood on the cross that men might have remission of their sins (Matt. 26:28). There is no sin which cannot be remitted if one correctly reaches the blood of Christ in obedience to Christ (Acts 2:38; Heb. 5:9). After one becomes a child of God, when he sins he has an advocate with the Father which is Jesus Christ the righteous (1 John 2:1). Through Jesus Christ as long as we remain in a covenant relationship with Him, His blood continues to cleanse us as we respond according to His will (1 John 1:7-10).
John raises two cases here in verse 16. One is praying for one whose sin is not unto death and praying for one whose sin is unto death. Prayer in this case is a measure of concern for one who is lost. You cannot pray and make someone spiritually right. This is a prayer of concern. I am taught to pray for those in authority and all men (1 Tim. 2:1,2). My prayers cannot make a man righteous without his will being involved. John is saying there are some whom you manifest a concern for by praying for them. On the other hand there are those who by rejecting Christ have no way to return because of the condition of their heart.
A case in point is Hebrews 6. Beginning in verse 4 the truth is presented about those whom it is impossible to bring back? Why is that the case? Because they have no desire to come back. We cannot read men’s hearts, only their actions. If the actions of the person is bent on never having anything to do with God any more, your prayer time has no value on them. It may help the Christian is terms of staying concerned but it has no value if the person cannot be touched by the goodness of God and His Word. Jeremiah was told by God to not pray for these people any more? Why did God say that? In that time of their spiritual life they were not listening to the law of Moses nor the prophets sent by God. This is the same sense of John in I John 5:16.