What religion became the most powerful during the emas

During the European Middle Ages, the Catholic Religion became the most powerful.

Explanation:

As the Roman empire felt the Cristian Catholic Religion adopted as the Empire Religion in the 300's by the Emperor Constantine became the most powerful Religious institution.  

The Church was the only institution that remained after the end of the empire.  

As the Empire collapsed by the several barbarians’ wars in 476. The church continued being the most important institution and continue to these days to be the most important Religious institution in the World.