Spread throughout the Roman Empire
Conditions in the Roman Empire facilitated the spread of new ideas. The empire's well-defined network of roads and waterways allowed for easier travel, while the Pax Romana made it safe to travel from one region to another. The government had encouraged inhabitants, especially those in urban areas, to learn Greek, and the common language allowed ideas to be more easily expressed and understood.[136] Unlike most religions in the Roman Empire, however, Christianity required its adherents to renounce all other gods, a practice adopted from Judaism (see Idolatry). The Christians' refusal to join pagan celebrations meant they were unable to participate in much of public life, which caused non-Christians–including government authorities–to fear that the Christians were angering the gods and thereby threatening the peace and prosperity of the Empire. The resulting persecutions were a defining feature of Christian self-understanding until Christianity was legalised in the 4th century.[137]
In 313, the struggles of the Early Church were lessened by the legalisation of Christianity by the Emperor Constantine I. In 380, Christianity became the state religion of the Roman Empire by the decree of the Emperor, which would persist until the fall of the Western Empire and, later, with the Eastern Roman Empire until the Fall of Constantinople. During this time (the period of the Seven Ecumenical Councils) there were considered five primary sees according to Eusebius: Rome, Constantinople, Antioch, Jerusalem and Alexandria, known as the Pentarchy.
[edit] Middle Ages
After the destruction of the western Roman Empire, the church in the West was a major factor in the preservation of classical civilisation, establishing monasteries and sending missionaries to convert the peoples of northern Europe as far as Ireland in the north. In the East, the Byzantine Empire preserved Orthodoxy well after the massive invasions of Islam in the mid-7th century. The invasions of Islam devastated three of the five Patriarchal sees, capturing Jerusalem first, then Alexandria, and then finally, in the mid-8th century, Antioch. The whole period of the next five centuries was dominated by the struggle between Christianity and Islam throughout the Mediterranean Basin. The battles of Poitiers and Toulouse preserved the Catholic west even though Rome itself was ravaged in 850 and Constantinople was besieged.