Christianizing The Roman Empire A.D. 100-400 by Ramsay MacMullen
Excellent study of how early Xianity spread, and I say that as a Fundamentalist Atheist. :P
This book is amazing in the way it shows how our ancestors were just like us today.
In the first chapter, masses of Bedouins flock to see Saint Symeon sitting on top of his pillar. With the same slippery Xian Bookeeping we see today, the church leaders of the time counted noses and put down every one of those Bedouins as a Xian convert. Then spread the story that umptaleen heathen Bedouin had miraculously converted to Jebus!
Well, those nomads didn't come to convert - they came to gawk at a nut sitting on a pillar. Just like some of us turn on our TVs today and laugh at Paul Crouch or Jimmy Swaggart. Then the Bedouin dispersed and continued happily worshipping Dusares (their male deity) and Allat (female deity, though her name sounds very familiar...).
MacMullen also notes one reason Xianity became so popular after Constantine got behind it - Xian clergy were tax-exempt and the state subsidized church construction. OTOH, the pagans still had to pay taxes. Then there were the miracle workers and the roving mobs of fanatics...
Warning: this is not light summer reading. MacMullen is the Dunham Professor of History and Classics at Yale University, and he...er, well, writes just like a Yale professor. His style is sometimes VERY dry. This thing has a whopping 43 pages of endnotes and author's commentary - for a book only 119 pages long in paperback.
Still, the book is well worth the effort and don't let me scare you away from it.
http://www.amazon.com/Christianizing-Roman-Empire-D-100-400/dp/0300036426