1John 5
“12 So whoever has God’s Son has life; whoever does not have his Son does not have life. 13 I write this to you who believe in the Son of God, so that you may know you have eternal life. 14 And we can be confident that he will listen to us whenever we ask him for anything in line with his will. 15 And if we know he is listening when we make our requests, we can be sure that he will give us what we ask for. 16 If you see a Christian brother or sister sinning in a way that does not lead to death, you should pray, and God will give that person life. But there is a sin that leads to death, and I am not saying you should pray for those who commit it. 17 Every wrong is sin, but not all sin leads to death.”
It’s important to take these snippets in context. The book of John is guiding Christians to be able to identify sin and the Law identifies it. In the very first chapter he tells us, “He is faithful to forgive us and to cleanse us from every wrong.”
He goes on to say,1 John 3: “23 And this is his commandment: We must believe in the name of his Son, Jesus Christ, and love one another, just as he commanded us. 24 Those who obey God’s commandments live in fellowship with him, and he with them. And we know he lives in us because the Holy Spirit lives in us.”
The only sin that leads to spiritual death is disbelief-it separates you from God.
I think this scripture is not nearly as clear in its meaning as you seem to think.
If disbelief is the sin Paul refers to here, this presumably means we should NOT pray for those who are unbelievers.
Which pretty much contradicts every sermon I've ever listened to where the point is made that we should pray to God to open the eyes of unbelievers and bring them to belief.
I also think disbelief, in the sense you use it, means more than absence of belief. I think it means believing and rejecting. The Bible says the demons believe and shudder. That's quite different from a lack of belief as such.
Perhaps we should call the absence of belief nonbelief and the belief/rejection anti-belief.