Early Christians were heavily persecuted.

The Christian Church traces its history to Jesus and the Twelve Apostles, and saw the early bishops of the Church as the successors of the Apostles in general. Central to the doctrines of the Roman Catholic, Orthodox and Anglican Churches is Apostolic Succession, the belief that the bishops are the spiritual successors of the original twelve apostles, through the historically unbroken chain of consecration.

From the beginning, Christians were subject to various persecutions. This involved even death for Christians such as Stephen and James, son of Zebedee. Larger-scale persecutions followed at the hands of the authorities of the Roman Empire, beginning with the year 64, when, as reported by the Roman historian Tacitus, the Emperor Nero blamed them for that year's great Fire of Rome. According to Church tradition, it was under Nero's persecution that early Church leaders Peter and Paul were each martyred in Rome. Further widespread persecutions of the Church occurred under nine subsequent Roman emperors including Domitian, Decius and Diocletian. From the year 150, Christian teachers began to produce theological and "apologetic" works aimed at defending the faith. These authors are known as the Church Fathers, and study of them is called Patristics. Notable early Fathers include Ignatius of Antioch, Polycarp, Justin Martyr, Irenaeus, Tertullian, Clement of Alexandria and Origen.

Christianity was legalized in the fourth century, when Constantine I issued the Edict of Milan in 313. Constantine was instrumental in the convocation of the First Council of Nicaea in 325, which sought to address the Arian heresy and formulated the Nicene Creed, which is still currently used by the Roman Catholic Church, Eastern Orthodoxy, Anglican Communion, and many Protestant churches.

On 27 February 380, Emperor Theodosius I enacted a law establishing Catholic Christianity as the official religion of the Roman Empire and ordering others to be called heretics. This period of history was also marked by the inauguration of a series of Ecumenical (worldwide) Christological Councils which established and formally codified critical elements of the theology of the Church. In 382, the Council of Rome set the Canon of the Bible, listing the accepted books of the Old Testament and the New Testament. Also, the Council of Ephesus in 431 declared that Jesus existed both as fully Man and fully God simultaneously, clarifying his status in the Trinity. The meaning of the Nicene Creed was also declared a permanent doctrine of the Church.