Facilitating Paradox mentions his current inspiration in Justo Gonzalez's The Story of Christianity (vol. 1). (Hear that Jim?)
I know "the Gonzalez" (as we always called it) is a classic textbook, and that it serves a particular function, but for my money and shelf space, I'm more partial to Linwood Urban's A Short History of Christian Thought — "short" meaning compact, not cursory.
It isn't the lightest of reading, but nearly every page has a topic that would stand alone, say, for a study group, and so it would be worth having for any serious lay student of Christian theology. Or, put another way, "If I could have only one book on the subject ..."
It isn't cleap new (lists at $31.95 for paperback) but it can be had for less than $4 if you get it used at places like Half.com.