When we pray, we are asking God for life. It pleases God that His children will ask Him for eternal life and the things that are necessary for physical life here. He has given us His Son already, why would He now withhold any good thing from us? We are instructed to pray for others as well, asking God for life for them. When we or others sin a sin that does not lead to death (physical or spiritual) we are instructed to pray for life for ourselves and them. It seems to be optional to pray for those who have sinned a sin unto death (“I am not saying you should pray for those who commit it”). ****
****A short devotional could not possibly cover this subject, but for two examples of a sin leading to physical death look at 1 Cor. 11 (those that are asleep) and Acts 5 (Ananias and Sapphira).
A sin leading to spiritual death (blasphemy against the Holy Spirit) Matt.12:3. Another is absolute total denial that God exists. If He doesn’t exist then how could He possibly forgive any of us?
But if we aren’t sure about those sins leading to death, we are not restricted from praying for those who have committed them, John just cannot give us an assurance that God will give life to those for whom we are praying.