Let me help you!
Before the fall of Constantinople, now Istanbul, Turkey, to the Ottoman Turks in 1453, the Christian realm is divided into two: to the west, the Roman Catholic church whose capital is the Vatican city and to the east, the Orthodox Catholic chuch whose capital si Constantinople.
When Constantinople fell to the hands of the Ottoman Turks (Ottoman Empire), the one true inheritor of the Christian realms became the Roman Catholic church lead by the Pope in the Vatican city, Rome, Italy since it was left unconteseted and since the Byzantine Empire has finally fallen.
I hope this answered your question :>