From Wikipedia

Christianity: The First Thousand Years

Christianity began in 1st century AD Jerusalem. Luke states in Acts 2:5-11 that there were present in Jerusalem on the day of Pentecost “devout men from every nation under heaven... Parthians, Medes, Elamites, (regions of present-day Iran), residents of Mesopotamia, Judea and Cappadocia, Pontus (both in Ancient Greece, present-day Turkey) and Arabians”. It spread initially in the Near East, ultimately becoming the state religion of Armenia in either 301 or 314, of Ethiopia in 325, of Georgia in 337, and then the state religion of the Roman Empire in 380. During the Age of Exploration (15th to 17th cent.), Christianity expanded throughout the world, becoming the world's largest religion.

Throughout its history, the religion has weathered schisms and theological disputes that have resulted in many distinct churches. The largest branches of Christianity are the Roman Catholic Church and the Eastern Orthodox Church, and the Protestant churches.

Early Christianity

The history of early Christianity spans from the death of Jesus Christ (c.26-36) to the First Council of Nicaea in 325.

The first part of the period, initiated by the Great Commission, during the lifetimes of the Twelve Apostles, is called the Apostolic Age. The earliest followers of Jesus composed an apocalyptic, Second Temple, Jewish sect, which historians refer to as Jewish Christianity. The Apostles and others following the Great Commission's decree to spread the teachings of Jesus to "all nations," had great success spreading the religion to gentiles, with Cornelius the Centurion traditionally considered the first. Early Christianity gradually, over several centuries, grew apart from Judaism and established itself as a predominantly gentile religion.

In the Ante-Nicene Period (literally before the First Council of Nicaea in 325), following the Apostolic Age, both incredible diversity and unifying characteristics lacking in the apostolic period emerged simultaneously. Part of the unifying trend was an increasingly harsh rejection of Judaism and Jewish practices. By the beginning of the Nicene period, the Christian faith had spread throughout Western Europe and the Mediterranean Basin, and to North Africa and the Near East.

The First Council of Nicaea (the origin of the term "Nicene") in 325 and the promotion of Christianity by Emperor Constantine I in the Roman Empire are commonly used to mark the end of early Christianity, beginning the era of the first seven Ecumenical Councils.

Constantine I and Christianity

The Emperor Constantine I was exposed to Christianity by his mother, Helena. There is scholarly controversy, however, as to whether Constantine adopted his mother's humble Christianity in his youth, or whether he adopted it gradually over the course of his life.

Head of Constantine's colossal statue at Musei Capitolini

Christian sources record that Constantine experienced a dramatic event in 312 at the Battle of Milvian Bridge, after which Constantine would claim the emperorship in the West. According to these sources, Constantine looked up to the sun before the battle and saw a cross of light above it, and with it the Greek words "ΕΝ ΤΟΥΤΩ ΝΙΚΑ" ("by this, conquer!", often rendered in the Latin "in hoc signo vinces"); Constantine commanded his troops to adorn their shields with a Christian symbol (the Chi-Ro), and thereafter they were victorious. How much Christianity Constantine adopted at this point is difficult to discern; most influential people in the empire, especially high military officials, were still pagan, and Constantine's rule exhibited at least a willingness to appease these factions. The Roman coins minted up to eight years subsequent to the battle still bore the images of Roman gods. Nonetheless, the accession of Constantine was a turning point for the Christian Church. After his victory, Constantine supported the Church financially, built various basilicas, granted privileges (e.g., exemption from certain taxes) to clergy, promoted Christians to high ranking offices, and returned property confiscated during the Great Persecution of Diocletian. Between 324 and 330, Constantine built, virtually from scratch, a new imperial capital at Byzantium on the Bosphorus (it came to be named for him: Constantinople)–the city employed overtly Christian architecture, contained churches within the city walls (unlike "old" Rome), and had no pagan temples. In accordance with the prevailing customs, Constantine was baptised on his deathbed.