> unless your argument is that mankind's idea of God also corresponds to the reality of what exists outside our heads
But all men do not agree on what God is at first place, or whether he exists or not. It doesn't make the idea of god less powerful(that's my point), however if you define "reality" as what men can experience, no men has ever experienced "God" and created a reproducible experiment of God. Therefore "God" cannot be a reality today. God is a philosophical question.
No, in that God is not (just) a philosophical question. There is a reality which exists. Either there is a God who actually exists in reality, or there is not. It is a question of what is the truth about what actually exists, not just a philosophical question.
Well, which/what "God"? the definition of God itself is purely a philosophical question. You can't ask yourself whether "God" exists or not, until you define precisely what you mean by "God". And men do not agree at all on what "God" is, even those that claim they follow the same religion. That's why you can't ask "science" to answer a question on a matter that has no formal definition to begin with. God is an idea first and foremost. a vague idea at best that bear no exact definition, thus a philosophy.
Mathematics is not the real world; it's a map, not the territory. But of course it's not surprising if a map resembles the real world...