1. What's the difference between ancient Christianity and medieval Christianity? What is the difference between Christianity in the Middle Ages and Christianity in the previous hundreds of years?

Ancient Christianity was a purely religious group. In the Middle Ages, it became a tool for the ruling class to rule the people (just like Buddhism in China) and an excuse and tool for war. For example, the Crusades in the Middle Ages took advantage of the relationship between Christianity and Islam, ostensibly fighting for God, but actually expanding territory, even including the later Children's Crusades, which is a good example.