A lot of us these days are on the atheist/agnostic end of the religiosity scale. So it can be easy to forget how important religion has been across history. Luckily there are plenty of books in the history of religion. Some of them are more general books about religious institutions impacted events, like the role of the Pope in the Crusades or the spread of Islam into North Africa. A few of them are specifically about the development of a religion over time. Those are the ones which are a bit more unusual, and if you are interested in a religion it's one of those that you probably want to find.