I would like to start this thread by saying that I do not mean to offend anyone and that it is not my intention to slam anyone's faith.
With that in mind . . .
I think we're all fairly familiar with the rise of Islam and the Crusades. The whole concept of the first few generations of Muslims conquering new territories in order to expand their faith so rapidly and in such a short period is one that has always fascinated me. Christianity never really did anything like that until much later in its history, i.e the Crusades, and certainly not on the scale that Islam did.
So the challenge is: With a POD no later than the reign of Theodosius, create a militaristic Christianity where its followers believe that they must expand their faith through force of arms.