Tom Holland is an historian and agnostic (as best I can tell). His book, Dominion: How the Christian Revolution Remade the World, released just last week, is astonishingly good.
While evangelicals (or Christians broadly) won't agree with everything he says (or assumes), his writing and scholarship are top notch. I can't remember the last time I had so much fun reading this sort of history.
Please get this in Logos, as it would benefit from the tagging and linking to ancient sources throughout the text.
(Publisher: Basic Books)