At what point in time though, would the concept have been thought out? When God was talking to himself in the creation, why was there not another form of putting it as one? A person does not usually talk about themself in the plural. Most references of God are in the plural though. When Moses saw the burning bush, it was considered the Angel of Adonai. It then says that when God saw Moses approaching the bush, there was a voice coming from the bush. When Moses realized it was God, he turned away because he was afraid to look at God. There seems to always be a voice, a physical presence that can be looked at, and then a fear that cannot be accounted for. Moses was not afraid to look at the burning bush itself, it is only the realization that God was represented that Moses had fear.
Jewish history points out that God walked with people, but in what form? God has the form of a spirit that can be heard. A physical form that can be seen. And a form that can be seen with only fear and dread of being consumed.
I do not see how Christianity could have "made it up". They did not form the thought of Judaism. Judaism rejects the "christian" view, which would seem to preclude that they "made it up".
Now some may assume that Christianity just replaced what was already "made up" by Judaism. I would like to say though, that early christians messed up. They should have totally destroyed all manuscripts of the former religion, if they were just replacing it. IMO, they were not replacing Judaism, neither were they adding to it. They saw the OT as an important Word from God.
It seems that Jesus was God in human flesh. The Holy Spirit was given to the Christian as God in the Spirit. God no longer walks with man, but lives inside. I take it a step farther than most and accept the Bible as a representation of God in the physical form. Not that the printed page has any power, nor the communion is God in the physical. It is what reading the Bible and taking of the communion represents. It is a physical form of God that can be approached by a human.
When God "speaks" it is usually not audible, but when one hears it, they will begin to understand that it is God who is communicating. They will then understand that God is to be feared, but one also knows that they will not be consumed by God, but will become like God originally created them to be. Hopefully that was not too meta for some.