That's a good point. Certainly, "God" isn't well-defined, so the theist has lots of vagueness to hide behind. Indeed, that might be the whole point of the concept of God, to challenge our cognitive hubris. Maybe there's a limit to our ability to understand things, and maybe God is supposed to stand for what's beyond that limit.

But in the case of Christianity, there's the story of Jesus which supplies Christians with a model to emulate. The prosperity gospel looks hypocritical and inauthentic because it's obviously the opposite of what the character Jesus would say or do, as represented by the gospels. That's the standard for an authentic Christian (a follower of Jesus), if anything is.

Generalization is generally unhelpful? Were you trying to be paradoxical? You generalized against generalization, so generalization appears to be fundamental to human thinking after all. That's the Humean sense I had in mind, which is at the root of all our concepts. If you're against that kind of generalization, you're against human thinking and understanding, which takes you to something like Zen mysticism or perhaps to a robotic kind of empiricism.