When I was working on my paper about the history of interpretation of the Star of Bethlehem, I used one source to help determine when natural explanations for the movement of the Star began, Charles François Dupuis. Why I did this deserves explanation as it provides some of the history of the interpretation of religions, the origins of Christianity, and how progress has been made since.
First off, even though you probably haven’t heard of Dupuis, you may be familiar with his methods as they are still used today in some fringe work, namely that of Acharya S (and I mentioned him and Acharya when debunking the version of the SoB from Zeitgeist). Dupuis lived and became prominent during the revolutionary period in France, and it was in 1795 that he published his massive work that tried to explain the origins of all religions. He argued that, ultimately, all religious practices could be traced back to interpretations of the movements of the heavens, namely the Sun, and all other religions were dependent upon antecedents of these myths. In particular then, Christianity would pick this up from the surrounding cultures, copying it and making Jesus simply another solar deity like Mithras.
Again, you haven’t heard of him, but he was a noted figure to one of the Founding Fathers in America, John Adams. In a letter exchange between him and Thomas Jefferson, Adams brings up Dupuis (spelled Du Puis) and insist that Jefferson become more acquainted with his work. Adams also called for a translation of Dupuis into all European languages to combat the “Corruptions of Christianity” (I think he speaks of its mythological and irrational components rather than irradiating the religion itself). I think Dupuis was also an influence on Thomas Paine, since he also talks about Jesus being nothing more than a Sun god.
However, the methods used by Dupuis are rather suspect. One sees a link between two things, therefore they are really much the same thing. This method was shown to be so flexible and worthless that back in 1836 Jean-Baptiste Pérès showed that Napoleon was a solar deity and never existed historically. A similar thing was done with another proponent of the all-is-the-sun-god argument, Max Müller, who was also shown by his own methods to be a solar deity and never existed (also talked about here and here). However, scholars such as Jonathan Z Smith are willing to set up Dupuis as a founding figure of the History of Religions School since they also had some similarity in method in their comparative religious practices.
As for why I cited Dupuis, this was because I expected him to use what arguments he knew of what could explain the Star in the sorts of rationalistic ways he employed, yet he does not make the Star a meteor or conjunction. Instead he dismisses it as folklore, as I discovered. That helped me demonstrate that before about 1800 no one had been trying to explain the Star as some naturalistic occurrence.
But when it comes to the ways we study religions, his intuitions were not really that bad, and we know that different cultures and religions influence each other. What was wrong with Dupuis was his methods of detecting and explaining that influence was inept and as flexible or more than Freudian psychology. But as psychology has advanced and become more scientific so has the understanding of cultural influence. Parallels cannot be just random coincidences but they need to be purposeful and interpretable to the peoples that crafted the myths. We can tell that Virgil used the works of Homer in crafting his The Aeneid, but there are lots of differences and reversals as well. The cultures need to be in contact, so one cannot take a symbol or story from India and suppose it explains a Celtic legend. This is all encompassed in syncretism, and it a universal and now much better explained phenomenon than in was in the 18th or 19th centuries. And such methods can be enlightening, including showing the origins and changes to the famous Greek myths.
What Dupuis shows is the dangers of what among Bible scholars is often called “parallelomania“, making connections no matter how weak. Sometimes a cigar is just not a Sun god. But some parallels are real, and we should not be afraid to point them out and argue that they are there because we may be mistaken. So when we see how Judaism originally did not have a view of differentiated afterlives, but then after contact with the Persians and Zoroastrianism they gained ideas of a fiery hell, a conflict of Good vs. Evil with Satan as the ultimate adversary, and bodily resurrection of the dead, we can be reasonably sure there was syncretism between Judaism and Zoroastrianism, the latter influencing the former. There was almost certainly the same thing going on in early Christianity, be it from mystery cults and their dying-and-rising gods to how we celebrate Christmas with trees and gift-giving. There are certainly differences between pagan and Christian beliefs, but there are also the similarities and differences that can be made intelligible.
But are we going to explain away all of religion as simply changing of ideas, or are there historical events that form the stories told? To tell how much history is in a myth, that will take some work.