I've been writing about historical immersion as a means to prepare to write historical fiction. The major points I've made have been about the tools to get historical information (primary and secondary sources) and getting rid of cultural and social prejudice. I've been using examples mostly from ancient Greece, and articulated in my book The Second Mission http://www.thesecondmission.com/. Today, I want to focus on religion. As with all cultural and social mores, we imagine the world was always the way it is for us today. We imagine our way of thinking and our ways of doing are the ways everyone lived and existed in the past. The truth is very far from that. Even today, people think much differently depending on their culture and society. One of the most important, if not the most important issue in all cultures is religion. Religion forms, focuses, and directs a culture. This is especially true in the ancient world. In a pagan worldview, which includes both animistic and pantheonic paganism, the culture understands the world and its forces only through the gods. There is no conception of natural forces or nature. In animism every thing that has force or life (plants, animals, heavens, clouds, the sea, bodies of water,...) has a god in it. The god sustains and produces the forces in nature. The god must be placated lest something bad happen. So whenever a creature is killed, the god within (or in charge of) the creature must be placated--thus all killing required sacrifice to appease the gods. With literacy comes pantheonic paganism. In this form of pagan worldview, the gods take on new meanings and responsibilities, but are still the focus of all the power in the world. Everything goes from the gods and the cause of all action in the universe is due to the gods. The new functions of the gods are related to civilization itself: wisdom, writing, music, metallurgy, war, love... With literacy new ideas spring fully armed from the minds of men. In the Greek worldview, men are fated (pathos) and the gods are fated (chronos), and the fates or both are not pleasant. The world revolves around the gods and fate. This is the center of the Greek universe. It is a universe where no one will make a move without a word from the gods. It is a world where everyone believes and those who don't, don't live long. The religion is forced and enforced. It is not coercive, it is obvious to everyone. Even the Greek philosophers did not disagree on this most basic view in Greek thought. They played on the edges of the whys and wherefores, but they did not dispute or disagree with any basic point of the pagan worldview.
Therefore, if you want to understand any culture well enough to write about it, you must begin to understand its view of religion. You can't hold a people's religious view in contempt. You can't judge it from within. You can only look at it within the context of their culture. It is a foundation and a center point of the entire culture, so you can't ignore it either. Many modern writers completely ignore the religious views of the culture they write about. They make their characters like modern thinkers who ignore religion or who parrot some non religious worldview. Their characters miraculously understand the world from a modern world view based in cause and effect. They assume that modern ideas infuse especially the thinking men and women of the past. This is a completely false and foolish idea about the past and about history in general. Religion drives the world--it especially powers the ancient world. The modern wars driven by Islam and secular Communism should warn us that the modern world is driven by much more religion than many would like to admit. In your immersion, you must learn much more than what the people ate (we saw religion played a huge role in Greek food) or what they did (look at the architecture of the Greeks, based on temples and the gods). You have to learn how they thought and why their thoughts became the reality of their culture. Only then can you start to understand enough to write about their history.