1 John 5:16 NIV
[16] If you see any brother or sister commit a sin that does not lead to death, you should pray and God will give them life. I refer to those whose sin does not lead to death. There is a sin that leads to death. I am not saying that you should pray about that.
When we see a brother or sister sinning we should first pray for them. Many people are quick to respond according to Matthew 18:5-17 and go to them and “point out their fault” but first we are to pray. There must be power in our prayers otherwise there would be not reason to be instructed to do so. Even if we can’t understand how it changes anything we should trust that it does.
This “sin that leads to death” could very well mean a sin that leads to actual physical death. Whether it be because what they are doing causes them to die or because they are so caught up in a sin that God calls them home. We can pray for that person to turn from their sin but praying for God to spare their life when they are dying because of their sin will not have the power to change things. Some people think it means spiritual death and the sin that leads to death is being blasphemy of the Holy Spirit. I don’t know for sure which one it is but if someone is blaspheming the Holy Spirit they are not a believer so they would not be a brother. John repeatedly uses the term brother to refer to fellow believers.
Always pray first and trust God with it all.