I've been working on a post for days on the history of the three basic kinds of Christianity:
Historic Eastern (Orthodoxy) which dominated the first Christian millenium,
Historic Western or Latin (Catholicism) which dominated the second Christian millenium; and
Reformation Christianity (Protestantism and all its many offshoots) which emerged in the middle of the 2nd millenium and is poised to become the largest form of Christianity in the course of this century.
I've done all kinds of number crunching and just needed to create the graphs showing the critical turning points but I'm working on a big project for January this morning so I haven't been able to finish the post yet .
In the meantime, here is an amazing 90 second depiction of the spread of the religions of the world which powerfully depicts how geographically constrained Christianity (as a whole) was until the Catholic expansion of the 16th century and then the beginning of the Protestant missionary movement in the 19th century.