The Byzantine emperor Constantine made it go viral around 300AD. He had a lot of attacks from external tribes on the leftovers of the Roman empire and a religio

The Byzantine emperor Constantine made it go viral around 300AD. He had a lot of attacks from external tribes on the leftovers of the Roman empire and a religion that teaches you to turn the other cheek was very helpful to adopt as a national religion. Then around 1050AD Catholicism split from Orthodoxy and if you were a ruler you got to pick between being in the same club under the influence of the Pope (with his perks and manipulations) or being Tsar with a patriarch underneath you. Some Eastern European countries tried both ways and settled on Orthodoxy. Western Europe rulers intermarried instead, and fought between each-other with the occasional help of the Pope club. They all wanted to claim some shred of grander legitimacy as descendent of the Holy Roman empire and absorb others, but it didn’t work out.