1 John 5:13-21, contains a message to Christians that one of their greatest blessings was the power to pray to God and have that prayer answered positively (if it is according to God’s will). That would include, the blessing and responsibility, to pray for a brother (presumable unrepentant) to give him time to come to his senses.
It is not God’s will that we pray for an erring brother who is engaged in a “sin unto death”. So it is incumbent on Christians to not only watch for sin but also to categorize the sin so we can know if we should pray for it or not.
The conclusion of this section states… “My little children, guard yourselves from idols”
Let me suggest that the sin we are not to pray for is the sin that a Christian commits when he rejects Jesus Christ and returns to idol worship. Note that the word idol in any of its forms does not occur anywhere else in this letter except at the end of this section on prayer for the erring Christian. That’s important to note… in fact it is screaming at us.
Review with me Nehemiah 9:18,
“Even when they had made for themselves a golden calf and said, ‘This is your God who brought you up out of Egypt,’ and had committed great blasphemies”
The children of Israel had witnessed all of God’s great miracles against the false God’s of Egypt and at their first opportunity rejected God by molding the image of a golden calf and worshipping it. The Holy Scripture characterizes that action as a “great blasphemy”.
It is also a great blasphemy for a Christian who has experienced all the great blessings that Jesus has to offer and reject those blessings and reject Jesus Christ and replace him with an idol.
Christians can pray and should pray for any other sin that might be committed by a brother in Christ but not the sin of blasphemy against Jesus Christ. If you do… don’t expect that prayer to be answered.