In the West, there was a pope, and in the East, there was a patriarch. The pope is the head of the Roman Catholic Church. He sort of serves as God's regent on Earth, and he doesn't answer to any secular ruler. And ever since the fall of Rome, there has been a lot of tension in Western Europe between popes and kings over who should have the real power. But in the Orthodox Church, they didn't have that problem because the patriarch was always appointed by the emperor. So it was pretty clear who had control over the church, so much that they even had a word for it, Caesaropapism, Caesar over Pope. But the fact that in Rome there was no emperor after 476 meant there was no one to challenge the pope, which would profoundly shape European history over the next, like, 1200 years.