I remember the most exciting book on this topic that I read, was the book on Constantin (the great) by Jacob Burkhardt. While maybe being a bit speculative, at least partly, I had the impression of highly plausible puzzle-solving by someone who actually read and understood the ancient sources. You will find a lot of details on questions like this. I find it most recommendable for anyone who is interested in (the making of) politics and religion now and then.