History of Religion: What is the best text available?

Looking for a good text that explains from where ideas in each religion came--tracing each to the oldest source(s) and putting them in context. Example: Zoroastrian and Buddhist influences on Christianity, etc. My goal is to become a better person and more respectful of others beliefs. I'm a former fundamentalist, turned agnostic (and very Buddhist), but I need some illumination on who influenced whom. What do you all recommend? Thanks so much for your help. =)