Among scholars of late antiquity and the early middle ages, a standard text is:

• Peter Brown, The Rise of Western Christendom (2003; 3d edn 2013).

Despite the title, Brown gives much wider geographic coverage. I think he's a beautiful writer, and he captures much of the texture and diversities of all the weird and wild Christianities and all the rock 'n' roll heretics and saints that tramped the earth for almost 1000 years. It's become a bit of a textbook at many universities, which is part of why it's now in its third edition. If cost is an issue, you should be able to find a good value on a used copy or an earlier run.