Tonus- I am not talking about a group of nomads hashing out the cause of lightning over a few evenings and coming up with a long explanation. Even today, we have a tendency to jump to conclusions on insufficient evidence in order to deal with things we may not understand.
This also points, then, to the conclusion having been instinctive. A conclusion that originates at the very beginning of mankind and manages to stand the test of time til today despite all the advances in comprehension.
Again, if there was a god who interacted with these people and guided them at some point, the development of religion would have begun along a single and clear track. With the certainty of god as a starting point, the answers would not have been made up or random.
This is a separate point, alluding specifically to the God of the Bible. The Bible, nevertheless, clearly states that God eventually became a mystery. In fact, it states that it wasn't until the time of Moses that he revealed his name again. It's no surprise that by then there were countless gods being worshipped just in Egypt alone.