Medieval history

We have become so accustomed to thinking of Christendom as an ideal body detached from all ties of political loyalty that it is well to be reminded that from the time of Constantine religious unity had stemmed in the first place from political unity. Religious unity could scarcely be thought of apart from political unity, if only because religious unity depended on some ultimate power of coercion. Hence all future medieval plans for the reunification of Christendom are fundamentally plans for political integration. (But) After the end of the eighth century Christendom was not longer politically united even in the most shadowy way...

- R.W. Southern, The Middle Ages, pg. 61