When the cultural conditions are in place, then the question whether Jesus was an actual figure, or an amalgam of figures, is already less interesting. The conditioning establishes the vacuum in which some will be drawn, and others will then be drawn to him/them.
It should not be a surprise (at all) when Jesus was an actual figure, but the exaggerations would be plenty. King Arthur experienced the same fate of getting much attributed to him.
Scrolls discovered in Egypt about Jesus, and that are different from the official storylines, appear to confirm that Jesus was an actual figure. His story was manipulated, and that is no surprise.
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Empires tend to fall because they are at the end of their course; they lack the innovation to move out of the harness that had been so successful for so long. It can happen to the best of them.
Yet the coincidence of Christianity and the Fall of the Roman Empire, meaning the popularization of Christianity inside and outside the Roman circles, cannot be denied. It changed the conditions in which the Roman Empire existed, inside and out.
The idea that there are many Gods puts people in a different relationship to one another than the idea that there is a single God. One system is based on a natural diversity in which the enemy without is overcome (or not), while the other system is based on unity in which the enemy can also exist within. Unity becomes a wobbly state, often after the greatest successes.
While historically not completely accurate, I highly suggest watching 'Agora' by Amenabar to get a feel about the clash between these different kinds of religions. Religion organizes society, so the form of religion organizes society in specific manners.
I mentioned in my other response to you today that it appears you focus more on concepts than on the actual structures contained within. I am mentioning this, hoping you will be interested in looking specifically for the structure contained in the social realities that humans built for themselves. They are the inner workings of a society.
