Religous Empires, Christendom and the Caliphate

Ok, so how big can the Caliphate get?
Also, how could Christendom ever unite as one nation
How would these two react to eachother if they existed at the same time?
 
Ok, so how big can the Caliphate get?
Also, how could Christendom ever unite as one nation
How would these two react to eachother if they existed at the same time?

I don't think that Christendom could unite as one nation. The rise of Islam was parallel to the rise of the Arabs, in that the religion was used as a unifier, and the Caliphate then spread through conquest, IIRC.
 
Well here's a map of the first Caliphate:

250px-Rashidun654wVassal.png
 
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