The early history of the church is still part of the New Testament story, and the canon of the New Testament and church doctrines did not take their present form until long after the Apostolic Church.

Traditionally, Pentecost is seen as the birth of the church, but despite the reports in the Acts of the Apostles of early mass conversions after Pentecost, the followers of Jesus Christ remained a small group or sect within Judaism, alongside the Pharisees, Sadducees and Essenes, until two decisive events turned their faith into a mass movement: the conversion of Paul, and the destruction of Jerusalem.

Paul's conversion on the road to Damascus is such a decisive event that in a real sense he might be said to be the founder of the church. Indeed, the name Christian was first applied to a group of believers in Damascus.

Christianity quickly spread through Damascus and Antioch, the capital of Syria and third city of the Empire, and on through Syria, Cilicia and Asia Minor. Later tradition would associate many churches with the early Apostles: Alexandria with Mark, both Antioch and Rome with Peter, Byzantium and the Scythians with Andrew, and Phrygia in Asia Minor with Philip - even the church in Persia and on the Malabar coast of India would claim it was founded by the Apostle Thomas.

The spread of early Christianity was due in part to the exodus of Jewish Christians to Asia Minor during the Jewish War in the years 66 to 70 A.D. But the first real missionary endeavours of the new movement were launched by Paul, whose journeys saw the church expand throughout the Eastern Mediterranean in what we know today as Cyprus, Turkey, Greece, into Malta, present-day Italy, and (perhaps) as far west as Spain.

The earliest followers of "The Way" were recruited in the synagogues, among the Jews of the Diaspora, and among ethical, monotheistic Gentiles who worshipped with Jews. For both groups, koine Greek was the common language, and their thoughts were shaped by the thinking of Plato and Aristotle. The sack of Jerusalem in the year 70 marked the end of the dominance of Jewish Christians in the church. Gentiles, who had achieved equality in the church through Paul's endeavours, now became the dominant Christians, and the focus switched from Jerusalem to the capital of the Gentile world, Rome.

The bridge between the New Testament story and church history is provided by the writers known collectively as the Apostolic Fathers, including Justin Martyr and the author of Clement at the end of the first century, and Polycarp of Smyrna and the authors of the Didache, at the beginning of the second century.

Justin Martyr, who was born of Greek parents in Palestine, saw a continuity between his Christian faith and his Greek philosophical past, and anchored his Christian faith in his Greek heritage. Polycarp, who is said to have known John the Divine, the author of the Book of Revelations, was the last living link between the Apostolic Church of the New Testament and the historic church of the Apostolic Fathers.

With the letter known as I Clement, written from Rome to Corinth around the year 96 AD, we begin to glimpse common patterns emerging in the liturgy, life and ministry of the church at the end of the first century. A clearer pattern of church order and ministry is defined in the early second century by Ignatius of Antioch in his writings.

As he was being taken to Rome to be martyred, he wrote seven letters setting out the threefold pattern of bishop, priest and deacon, with the local bishop as the focus of unity in the face of schism and heresy.

By the beginning of its second century, Christianity was under attack, internally and externally, from a number of diverse, competing sects known collectively as Gnostics, who claimed access to secret knowledge (gnosis). For Gnostics, the spirit was good and the flesh was evil, and they believed in a remote supreme god, sometimes identified with the God of the Old Testament but who was disengaged from the world.

The first firm challenge to heresy within the early church came from Irenaeus, the author of Against Heresies. A Greek who had learned at the feet of Polycarp before moving to Lyons, he became the first bishop in Gaul (France).

The challenge from Gnosticism and other heresies also led to the church agreeing on the canon of Scripture, deciding which books were to be included and which excluded from an accepted Bible. Irenaeus was among the first to talk about a New Testament scripture alongside the Old Testament.

Apostolic teaching, handed down through successive generations, and apostolic scripture, in the agreed books, amounted to the common apostolic tradition shared in common by an increasingly diffuse and diverse church, now scattered throughout the empire and beyond.

The challenge of heresy and schism also marks the beginning of theology, and Tertullian, the North African who died in 220 AD, is regarded as the father of Latin, western theology, although he later became disillusioned with the mainstream church.

North Africa produced other great theologians at the turn of the second and third century, including Clement of Alexandria, Origen (also born in Alexandria), and Cyprian, the martyr bishop of Carthage.

Apart from heresy and schism, the church also faced regular persecution, often for the refusal of Christians to take part in the emperor cult, to swear oaths or serve in the imperial army, but also because of widespread vulgar charges, originating in eucharistic practice and the teaching of Christian love, that Christians indulged in cannibalism and incest.

During the severe persecution under Marcus Aurelius in 177 AD, Tertullian could comment with sarcasm: "If the Tiber rises too high or the Nile too low, the cry is `The Christians to the lion'. All of them, to a single lion?" Despite persecution and martyrdom, and Tertullian observed, "the blood of the martyrs is the seed of the church".

The church was thriving, and missionary, social and intellectual advances were preparing the way that would lead to the conversion of the Emperor Constantine at the beginning of the fourth century, the accommodation of the church with temporal power, and the consolidation of church teachings at the great ecumenical councils in the decades that followed.

But the old heresies, schisms and battles would not go away. The theories and beliefs of Gnostics and Arians would continue to resurface in the church in successive generations, and they continue to appear today. The rift between the Greek East and Latin West would widen throughout the remaining centuries of the first millennium, so that the church, despite winning the internal battle for orthodoxy, could never succeed in maintaining its unity or a common church order.

The divisions of the 21st century can be traced back to the seeds sown in the first, second and third centuries of church history.

Patrick Comerford is an Irish Times journalist and a writer on theology and church history. Contact: theology@ireland.com