—1 John 5:16-17 (NASB)
This can be a tough verse because John doesn't specify which is a sin "leading to death" and which is "not leading to death". And it grabs our attention because John actually says not to pray for those committing a sin "leading to death"? As with all verses both difficult and easy, the number one rule for proper biblical interpretation is to always look at what other Scripture has to say. It's never a good thing to base a doctrine on a single verse, but on what the Bible has to say about a topic as a whole. So what we need to do is look for anything related in the rest of God's Word.
So has there ever been an instance in the Bible where someone was told NOT to pray for someone? Yes, actually — Jeremiah. (Jer. 7:16; Jer. 11:14; Jer. 14:11) God told Him NOT to pray for the people. The fact is that they were so steeped in rebellion that the only thing left for them was God's judgment. Another instance occurs in Ezekiel (Eze. 14:14; Eze. 14:30) . God explains to Ezekiel that the people's rebellion was so bad that…
—Ezekiel 14:20 (NASB)
The people had rebelled against God to the point that not even the prayers of the three most righteous men in history could save them.
Now this reveals to us what particular sin it is which "leads to death": rebellion. Now I'm not talking about the personal rebeliion as typified by the Prodigal Son, but corporate rebellion like making a golden calf or embracing false gods in place of the One True God. The Old Testament is replete with examples of how God passed the death sentence on those who rebelled in such ways. In fact, this is at the very heart of the matter when he first destroyed the world by flood.
So how does this apply to us today? Well, we should always intercede on others' behalf when they're acting the part of the Prodigal Son, but not when they're leading en masse to other gods. We might pray that Benny Hinn becomes ineffective and his so-called ministry decimated, but to pray for him personally? I don't think so. Those leading rebellion are in a completely different category.†††