How is it that mankind came to the idea of God?  I think I would like to speculate, with the understanding that whatever it was it happened in prehistory so there is no way we can really know.

I don't buy the "awe" theory nor the "seeking for explanations" theory.  I think these entered the story after the fact.

When we are in high school biology, the second or third chapter of our textbook tries to define what "life" is, and draws the distinction between life and non-life.  This is so ingrained into our culture that we take it for granted.  But it is a scientific observation, and prior to the eighteenth and nineteenth century the line was nowhere near so clear, so we had maggots appearing from non-life spontaneously, and so on.

Of course, we are also more than just "alive." We are also sentient (experience rather than just detect and measure) the outside world and our insides (sensations such as hunger), and, further, we experience feelings and desires (often driven by instincts, such as fear or anger).  Beyond being sentient, we are also conscious -- we "know" we exist and we know what we sense and we can infer all sorts of things.

We also assume pretty much automatically that everyone we know is similar to us, being alive, sentient, and conscious.  Why should we not draw a similar conclusion about the external world?  Why should we not just assume that other animals, and things like mountains and rivers and trees and forests and clouds and so on are like us?  To be sure they look different, and generally don't behave as humans do, but then each person is different too.

So, it would seem to me it behooves me, at a minimum, to try to get along with all these living, sentient, conscious things in our environment.  We have behaviors that indicate politeness, so be polite to the big tree and greet it properly, and when you kill an animal, ask its spirit for forgiveness.  Just as other humans we hurt can get revenge, the mountain may do so also (in it's mountain way) if we insult it or neglect it.  It may be slow witted, and may be very patient, but, regardless, it is in our interest to try to get along.

There is a passage in Cato's ancient book on agriculture about what to do if you have to remove a stand of trees -- it is important that the spirit or spirits of that stand be propitiated and taken care of.  For this he describes a set of rituals.

Now the Romans were not animists in the more "primitive" sort of way -- they had an organized pantheon of deities, but they also still had this assumption that there is consciousness out there other than just in ourselves.  A pantheon of deities with specialties would, with this frame of mind, arrive quite soon in human development -- things like the sky, the weather, the oceans, and so on, are universal and not local, and would naturally be seen to be far more important than the spirits of a stand of trees.

Then there would be speculation about where all this consciousness comes from, and inevitably the more introspective would consider a single "high god," not part of the world but its creator or sustainer.  In short, from the very first, human religion would be animist, "pagan," and monotheistic going in.