I happen to believe in the whole Christ/God deal, but that's just me.
However, I have struggled with that faith and in doing so have turned to concepts such as secular humanism, atheism and agnosticism throughout my life, so I'll try to offer my opinion on that question from those earlier mindsets.
When I was an atheist, I assumed that God was born out of that fear of the unknown that you were talking about. Humanity has always had a penchant for creativity, for storytelling, and I believed at one time that God came out of that as a response to that fear of the unknown and fear of what might be out there waiting for us.
I mean, most of us can't even stand waiting to find out what we got for our birthdays. How are we expected to wait and see on something as terrifying as eternity and whatever the hell that might entail?
I also used to believe that the concept of a creator or creator(s) came out of a time when science was incapable of explaining just about everything that made the world and universe as we understood it capable of running, existing and so on. It made sense to create something to answer those questions, to deal with that fear of the unknown and to establish a concept of morality to draw the line from point A., being alive, out and about to point B., being dead.
That was also my answer to different cultures having a different God or even more than one. Although many similarities bind both the cultures and peoples of today and those of the years and centuries gone by, there are also enough differences to demand that each culture, society, whatever have an answer (of sorts) that would work for their own ideas of eternity, morality, the unknown, the spirit world and everything in between and all around it.
I believed that the first concepts of God came out of the first time we ever began to seriously let these questions get to us, and you can probably go back as far as you want for something like that.
I don't believe that anymore, but that used to be more or less my explanation for it.
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