1 John 5:16
If anyone sees his brother committing a sin not leading to death, he shall ask, and God will give him life—to those who commit sins that do not lead to death. There is sin that leads to death; I do not say that one should pray for that.
What I mean is like this :
One day, Mr.X say that "my brother committed sin that leads to death".
Then the following condition is interested me, which is : "then we don't have to pray for that"
This question raised because (to me) it seems human won't ever be able to decide that someone committed sin that leads to death ---> the result then there never and have ever occurred when someone tell someone not to pray for any kind of sin.
A. I think a faithful and loving person - will never give up to pray for someone (let alone this someone is his brother) no matter how "crazy sick" sin this someone committed.
But reading the verse above,
B. it seems the verse do teach to just give up to pray about it when he/she sees that his/her brother/sister committed sin that leads to death.
My point A contradict point B.
Based on point-B, I need to correct my point A. Which becomes :
C. Although someone is a faithful and loving person, there is a limit where this someone doesn't need to pray if he sees his/her brother/sister committed sin that leads to death.
But still, it seems point-C is useless. Because (I think) it seems impossible that human (let alone the faithful and the loving person) dare enough to decide "oww... he/she committed a mortal sin !".
The question is :
Am I correct to think that the verse encourage "yes... you can decide if it's a mortal sin or not".
If I'm wrong, how is the Christian interpretation of this verse regarding my description above ?
PS : I don't put my question to a specific denomination yet. If it can not be answered then I will edit it.
Based from Sola Gratia answer, I have a new verse :
If your brother or sister sins, go and point out their fault, just between the two of you. If they listen to you, you have won them over.
Me : in this stage, it's not useless to pray for him
But if they will not listen, take one or two others along, so that 'every matter may be established by the testimony of two or three witnesses
Me : in this stage, it's still useful to pray for him.
If they still refuse to listen, tell it to the church; and if they refuse to listen even to the church, treat them as you would a pagan or a tax collector
Me : in this stage, this brother committed mortal sin. It's useless to pray for him. In other words, there's no need to hope that God will forgive him - no need to hope him to repent.
So... based on Sola Gratia's answer, does 1 John 5:16 means something like the above situation when a faithful and loving person needs to give up praying for someone ?