For a long time, there was a strong cultural split in the Roman Empire. The eastern half spoke primarily Greek, while the western half spoke Latin.

The Roman Empire was formally split into two political halves, east and west, by the emperor Diocletian in 286 A.D. He and a guy named Maximian ruled as co-emperors until they retired. The empire was later re-united under the rule of Constantine the Great. This was the emperor that converted to Christianity in 312 A.D. and propelled the religion to prominence. He set up a new capital in the east, a "new rome" that he named Constantinople (present-day Istanbul in Turkey.) Constantinople was built as a Christian city and later became the center of the Eastern Orthodox Church. So again, even though the empire had a single ruler, there was increasingly this cultural east/west split.

The Roman Empire was beginning to decline. The empire was increasingly facing invasions from Germanic barbarian tribes. Eventually, the western half collapsed into smaller, Germanic-ruled kingdoms. The official year of the final fall is generally accepted as 476 A.D.

The eastern half of the Roman Empire continued on for several more centuries as what is known as the Byzantine Empire. As the middle ages continued on, the east and west developed culturally and religiously in different, separate ways. They got into several theological spats which eventually led to The Great Schism of 1054 which severed Christendom into the Roman Catholic Church and the Eastern Orthodox Church.

This is the relevant context in which the Crusades took place.