What was the substance of his faith? Believing that a god, or even The God of the bible exists does nothing for a man. He was aware that it was sin to go with the king into the King's pagan temple:
2 Kings 5:18 But may the Lord forgive your servant for this one thing: When my master enters the temple of Rimmon to bow down and he is leaning on my arm and I have to bow there also—when I bow down in the temple of Rimmon, may the Lord forgive your servant for this.”
So, this seems to suggest that he was aware of the Law and had knowledge of God that we are unaware of, just as with Rahab. The bible doesn't give every single detail of what people knew, but the fact they (rahab, naaman, etc) means that they knew more than just "the God of Israel exists" and there is no indication that anyone has ever been saved apart from the gospel at any point.
There was no Gospel before the Law, that's why Paul said, "sin is not imputed where there is no Law."
we have so much knowledge of God now that we can't imagine what it was like 4-5000 years ago when there was only God's creation for the Gentiles to recognize.