At first sight, Christianity today seems very different from the beliefs and practices of Jesus of Nazareth and His first followers 2000 years ago. Roman Catholicism, Eastern Orthodoxy, Anglicanism, the Protestant churches of the Reformation, the liberal Christianity of the modern world, Pentecostalism - now there almost seem to be Christianities, in the plural, rather than a single faith.

If we look back over two millennia to see how this came about, we witness a drama which unfolded in several acts. Indeed, Christian history is even more complex than that, for none of these acts ends neatly, leading on to the next act. Rather, each of them continues, side by side with its successor, on a stage which becomes increasingly crowded, with many different levels.

Jesus and His first followers were Jews, living in a rural area of a land which, though under Roman occupation, had been shaped by yet another two millennia of history, the history of the people of Israel. Their thinking was stamped by Jewish faith, in one God who had brought His people out of Egypt and given them a promised land, with a king, a temple and a holy city, but who had allowed all these to be taken away by foreign conquerors, inflicting great suffering on them.

Out of this suffering came a new faith, which included the expectation of an end to the present world by God's intervention and the coming of His Messiah. All these expectations come vividly to life within the pages of the New Testament and stamped the character of earliest, Jewish Christianity.

However, this earliest Jewish Christianity soon suffered a devastating setback: Jerusalem was again destroyed only 40 years after the crucifixion of Jesus. In any case, there was a growing rift between Christians and Jews as, through the work of missionaries like Paul, Christianity spread into the wider Greek world, and into urban as well as rural areas.

Under the influence of Greek philosophy, Christian thinking increasingly adopted Greek concepts and developed great abstract systems, not without conflicts. "Heresies" kept splitting the church, which was constantly in conflict, making it necessary for emperors from Constantine the Great onwards to hold councils intended to keep the peace as well as to define true doctrine. Christianity spread over the East, from Jerusalem, Alexandria and Antioch to Constantinople - the later Byzantium - and on to Kiev and Moscow, but in many ways it was a divided Christianity. The complex history of the Hellenistic, Greek and Russian church is an act in the drama which is still continuing.

Alongside it developed another act of the drama, as in the West Rome gained a supremacy over other churches, and the Pope came to be seen as the head of the true church, the Roman Catholic Church. During the Middle Ages this church became a formidable institution, strong enough to take on even the emperor, and wielding immense power which was demonstrated not least in the Inquisition.

Inevitably corrupted by that power, this church was clearly in need of reformation. However, reform made its mark not within the church but through the movement led by Martin Luther in the 15th century, followed by Calvin and others, producing the churches of the Reformation: Lutheran, Calvinist, Methodist, Presbyterian and others.

Another act had begun which is still continuing today. But now the modern world was in the making, with the 18th-century Enlightenment, and all forms of Christianity were under challenge from philosophy and the rising sciences, which posed a threat to the fundamentals of religious belief. The subsequent industrial and technological revolutions represent yet another act in the ongoing drama and one of which we are very conscious today.

However, this is not yet the end. The sciences not only posed a threat to religion but also, especially in the form of archaeology and history, did much to shed new light on the origins and growth of Christianity; indeed it is the sciences which make this portrayal of Christianity possible. And there is a growing awareness that we live, not in a modern but in a post-modern age, in which Christianity may yet, for all its complex and troubled history, find a new place.

It has to be accepted that over this long and complex history there have been many deviations from the original essence of Christianity. There have been gruesome aberrations and signs of decadence, monstrous crimes and blasphemies committed by Christians. One need only recall the persecutions of the Jews and the heresy hunts; the "holy" wars and the burning of witches, the wars of religion and all the other crimes committed in the name of Christianity.

However, here the essence of Christianity keeps breaking through despite all the perversions. That raises a question which it is difficult to get out of one's head. Why has this Christianity kept surviving despite all the un-Christian elements in its history? For like a great river which has a modest beginning somewhere and has kept making new cuts through the emergent landscape, Christianity has kept inserting itself into ever-new regions.

In so doing it has experienced violent rejections and undergone revolutions, indeed has itself often caused new shifts in world history. But mustn't we also see here a stream of goodness, mercy, readiness to help, care, which flows from the source, from the gospel, through history? Granted, an infinite amount of debris, flotsam, silt and rubbish has been collected on the long way through the centuries. But has the water at the spring really become polluted, as many people say?

If that is so, how is it that the essence of Christianity did not get lost, but can be recognised time and again: Jesus Christ, His words and actions, His life and death, as an orientation, criterion, model for the concrete life of the individual and the community of faith, for relations with fellow human beings, human society and finally with God?

It is remarkable that time and again the spirit of Jesus of Nazareth has managed to establish itself when persons, institutions and constitutions, have failed, wherever there have not just been words, but quite practical discipleship; for the truth of Christianity is not just knowledge of the truth but existential truth.

So how is it that neither pagan emperors nor "Christian" dictators, neither power-hungry Popes nor dark inquisitors, neither worldly bishops nor fanatical theologians have been able to quench this spirit? Why could the hierarchy never completely veil mutual service of Christians in love, dogmatics never fully veil the discipleship of Christ?

What is there about this spirit, that all down the centuries, in an unparalleled movement, it has continually motivated, indeed driven people to break down all the cultural, social, political and religious fortifications, and take seriously the earliest Christian ideal of a love for the neighbour and even the enemy?

It is a strange historical mystery: monks and saints of the early church appear alongside court theologians and court bishops, Francis of Assisi alongside Innocent III and Boniface VII, Martin Luther alongside Leo X, Catherine of Siena and Teresa of Avila alongside the Grand Inquisitor, Blaise Pascal in the middle of French absolutism, William Booth in the depth of Victorian poverty, Karl Barth, Dietrich Bonhoeffer and Alfred Delp in resistance to the Christianity of a bourgeois culture and National Socialism - not to mention figures in our day like John XXIII, Willem Visser't Hooft, Martin Luther King, Helder Camara and Mother Teresa.

All these known names simply stand as representatives of the countless unknowns whose names are not listed in any church history yet who nevertheless make up the hidden power of Christianity, its true spiritual history. They are representatives of that faith movement consisting of those countless unknowns down the centuries who have gone by the values, criteria and attitudes of the man from Nazareth, who have learned from them that the blessed are those who are poor before God, who do no violence, who hunger and thirst for righteousness, who are merciful, make peace and will be persecuted for righteousness' sake; who have learned from him to pay heed and to share, to be able to forgive and to repent, to be sparing, practise renunciation and offer help.

To the present day they show that where Christianity really goes by its Christ and allows him to give it strength, it can offer a spiritual home, a place of faith, hope and love. Time and again they show in the everyday world that supreme values, unconditional norms, deepest motivations and highest ideals can be lived out, indeed that from the depths of belief in Christ suffering and guilt, despair and anxiety can also be overcome.

No, this faith in Christ is no mere otherworldly consolation but a basis for protest and resistance against unjust situations here and now, supported and strengthened by a restless longing for the "wholly Other".

Granted, this often hidden history of Christianity is as uninteresting to die-hard critics of Christianity as it is to certain journalists, hot-foot after the sensation of the day. After all, it is much easier to report on a scandal involving a bishop, or a papal visit, than on pastors in the parishes, wearing themselves out in the service of young and old, and still performing this service with a joyful heart and head held high.

But it is precisely these men and women, whether ordained or not, who continue the cause of Jesus Christ. Indeed, there have always been times when little of true Christianity was to be seen in the life and activities of hierarchy and theologians, but when nevertheless those countless, mostly unknown, Christian "little people", (but always including some bishops, theologians and particularly members of the parish clergy and religious orders) were there to keep alive the spirit of Jesus Christ.

And what kind of a spirit, what kind of a power is it that is at work everywhere? Is everything mere chance, mere fate? No. For believing Christians, beyond doubt more is involved here. For them it is clear that this effective spirit of Jesus Christ is not an unholy human spirit but the Holy Spirit, the spirit, the power and might of God: God's spiritual presence in the heart of believers and so also in the community of faith.

This spirit sees to it that there is not just research, information and teaching about Christianity but that Christianity is experienced with the heart and also really lived out and put into practice - for good or ill, since that is human nature, and in trust in this spirit of God.

So Christians may be sure that Christianity has a future even in the third millennium after Christ; that this community of the spirit and faith has its own kind of "infallibility". However, this does not mean that some authorities in particular situations do not make mistakes or perpetrate errors, but rather, that despite all mistakes and errors, sins and vices, the community of believers will be maintained by the Spirit in the truth of Jesus Christ.

In a strange way one feels reminded of the famous advice of the Pharisee Gamaliel, a contemporary of Jesus, who was a Jewish teacher of the Law respected by all the people. At any rate according to the account in the Acts of the Apostles, after the arrest of the apostles he is said to have remarked to the "supreme council" in Jerusalem about such Christians; "if this plan or this undertaking is of men, it will fail; but if it is of God, you will not be able to overthrow them. You might even be found to be opposing God."

Father Hans Kung is retired Professor of Dogmatic and Ecumenical Studies at the University of Tubingen, Germany. His publications include Justification (1957), Infallible? (1971), On Being a Christian (1974), Global Responsibility (1991), Judaism (1992),Christianity (1995). In 1979 the Vatican ruled that Father Kung could no longer be considered a Catholic theologian or function as such.