Alister McGrath is the Andreas Idreos Professor of Science and Religion at Oxford University.

To its many supporters, the Protestant Reformation represented a necessary correction and long overdue renewal of the Christian faith, liberating it from its imprisonment to the transient medieval intellectual and social order, and preparing it for new challenges, as western Europe emerged from the feudalism of the Middle Ages.

Christianity was being born all over again, with a new potency and capacity to engage with an emerging new world order. Yet from its outset, the movement was seen by its critics as a menacing development, opening the way to religious mayhem, social disintegration and political chaos.

What I want to consider here is one of the most significant and distinct Protestant beliefs which emerged from the Reformation, which proved to be charged with revolutionary promise, creating new conceptual and practical possibilities that had the potential to change the face of Christianity.

The belief I want to consider is the nature of the church - above all, what establishes the Christian credentials of a community of faith.

The marks of the church

The medieval church in western Europe offered a strongly institutionalized account of how salvation was effected. There was no salvation outside the institution of the church; it was by membership of the sacral community and observation of its rites that the individual secured salvation. Continuity with the apostles was safeguarded by historical institutional continuity, transmitted by the laying on of hands, passed down from one generation of the successors of the apostles to the next.

This strongly institutionalized vision of the church was often defended by citing a maxim of the third-century martyr Cyprian of Carthage: "outside the church, there is no salvation." If you wanted to be saved, you had to belong to the Catholic church.

The forced detachment of the fledgling Protestant churches from this body, which began in the 1520s, was thus fraught with theological peril. Were these breakaway communities really Christian churches? Could they really offer the same assurance of salvation and spiritual security as the Catholic church? These were no academic questions, but matters of ultimate significance. Salvation was a serious matter in the sixteenth century.

The Protestant response to these entirely proper questions was to offer a new vision of what it meant to be a "Christian church" that removed any necessity for institutional continuity with the medieval church. While tokens of historical continuity with the apostolic church were to be welcomed, the all-important thing was continuity with the apostolic teaching.

This opened the way for one of the most characteristic features of modern Protestantism: the radical proliferation of "churches" in the modern period. It laid the conceptual foundations for a way of thinking about the nature of the church that encouraged entrepreneurs to set up their own congregations, or even denominations, where necessary breaking away from older communities. There might well be one universal church, to which all Christians belonged - but there were countless local churches, each embodying a distinctive idea of what it meant to be a Christian, often adapted to the specifics of a local cultural situation, or shaped by a controversy whose theological relevance and emotional potency dwindled with each passing year.

A consensus began to emerge that there were two - and only two - essential elements of a Christian church: the preaching of the word of God, and the proper administration of the sacraments. As Calvin put it:

"Wherever we see the Word of God purely preached and listened to, and the sacraments administered according to Christ's institution, it is in no way to be doubted that a church of God exists. For his promise cannot fail: 'Wherever two or three are gathered in my name, there I am in the midst of them' (Matthew 18:20)."

Calvin's definition is significant as much for what it does not say as for what it does explicitly affirm. There is no reference to the necessity of any historical or institutional continuity with the apostles. For Calvin, it was more important to teach what the apostles taught than to be able to show an unbroken line of institutional continuity with them. (It should be remembered here that Calvin himself was never really "ordained"; he was simply licensed as a pastor by the city council of Geneva.)

For Calvin, institutional continuity was not sufficient to guarantee intellectual fidelity. For Calvin, the medieval Catholic church had suffered from institutional drift, losing its grounding in the fundamental ideas of the apostles, which were, of course, expressed in the Bible.

This radical new understanding of the church in effect envisaged the church as a community which gathered around the preaching of the word of God, and celebrated and proclaimed the gospel through the sacraments. Where the gospel is truly preached, there a church will gather. Protestant theologians, sensitive to the charge that this new approach represented a distortion of a proper theology of the church, pointed to a classic statement of the first-century Christian writer Ignatius of Antioch: "wherever Christ is, there is also the church (ubi Christus ibi ecclesia)." Gathering together in the name of Christ ensures his presence - and with that presence, a church comes into being.

Denominational proliferation

Although this distinctive and characteristic Protestant doctrine of the church dates from the early 1520s, its full significance only became clear in the nineteenth and twentieth centuries. The absence of a universally accepted authority structure within Protestantism meant that enterprising individuals, often fired up by a vision for a specific form of ministry, could start their own congregations, or even their own denominations. The outcome of this was inevitable - the emergence of a consumerist mentality, through which Protestants felt able to pick and choose the local church that suited their needs, beliefs or aspirations. And if they didn't find one that was just right, they would establish their own. Catholic critics of Protestantism often point to its innate fissiparous tendencies, which they regard as showing a lack of concern for the fundamental unity of the church.

While this congregational inflationism is unquestionably problematic, it is essential to appreciate that, in the eyes of some, it has two fundamental strengths, both of which are of decisive importance for the shaping of today's global Protestantism:

It allows Protestantism to deal with rapid social and cultural change, which often leads to churches being locked into the realities of a bygone age. Entrepreneurial pastors and preachers can easily recast a vision of the gospel, adapted to the new situation - in which the same way as older visions were adapted to their situations - and thus prevent Protestantism being trapped in a time warp. It enables Protestants to respond to perceived needs for specialist ministries to specific groups through the formation of voluntary societies, which often come to exercise a para-church role. It enables Protestant churches to deal with situations in which the denominational leadership is seen to be radically out of touch with its membership - typically, by pursuing theological agendas or cultural trends which are not accepted by the majority of their congregations. It does not matter whether these agendas are right-wing or left-wing, conservative or liberal. Protestantism empowers the congregation: first to protest against their leaders; second, to remove them; and third, to form their own congregation elsewhere, while still remaining a Christian church. While some Protestant denominations attempt to shield themselves against such accountability to their membership, these fundamental rights remain, in principle, as part of the movement's core identity. A Protestant believer can leave one denomination and join another, while still remaining a Protestant.

It is not unfair to suggest that the Protestant vision of the church unleashes a Darwinian process of competition and survival, gradually eliminating maladapted churches, and ensuring that what survives is well suited to the needs and opportunities of the day.

Mainline Protestantism in the United States is undergoing significant changes at the moment, as it tries to adjust to changes in America's social dynamic. This has meant the painstaking, and at times painful, re-examination of accepted understandings of what it means to be a Protestant church in a rapidly changing society.

In 1929, H. Richard Niebuhr published The Social Sources of Denominationalism, a study of the origin of the modern American religious denomination. Niebuhr's analysis pointed to the continuing importance of denominationalism within the United States, in that it was sustained by deeper social causes. To belong to a specific denomination was often to make a statement about historical origins or social status, as much as theological beliefs. The future of the denominations in their present forms seemed secure.

Yet from the 1960s onwards, the limitations of the traditional denomination were becoming increasingly clear. Some strongly entrepreneurial Protestants found themselves more and more frustrated by the institutional inertia of denominational structures, which increasingly appeared to them as unresponsive bureaucracies, uninterested in local initiatives or innovations.

Frustration with denominational structures is not, of course, anything particularly new. The great Protestant preacher Harry Emerson Fosdick, who played such an important role in the great fundamentalist controversies of the 1920s, once made the astonishing revelation that he had once considered leaving "the historic Christian organizations" in order to start his own "independent movement." Fosdick was dismissive of those who demanded ecclesiastical loyalty, holding that his only loyalty was to Christ. Yet despite his frustrations, he never set up his own church, even though his personal reputation was such that its future would have been secure.

Yet since the 1960s, American Protestantism has been increasingly characterized by the growth of market-shaped or market-driven congregations, led by strongly entrepreneurial individuals. Their sense of theological vision, coupled with a "can do" mentality that was nourished and inspired by the Protestant work ethic, eventually drove them to achieve their goals outside the structures of traditional denominations, especially in the aftermath of the theological and cultural turmoil of the 1960s. Like Martin Luther, they did not particularly want to work outside their mother churches - but the needs and realities of the new cultural situation seemed to provide them with no alternatives.

The outcome was a surge of new initiatives, meeting needs which were held to be largely ignored by mainline denominations, and setting new patterns for how churches work, develop and organize themselves.

The future of the church?

My core thesis here is that these innovative developments are given theological legitimization by the Protestant understandings of the church which emerged at the Reformation, which located its identity as a Christian body not in its institutional history or connections, but in its fidelity in preaching and ministering the sacraments. This proved to be a radical, dangerous idea, with the capacity to cause disintegration on the one hand, and renewal on the other.

As obvious examples of these developments, we might cite Calvary Chapel, the Vineyard Churches and the Willow Creek network. Yet we must also note the phenomenon of the "community church," which allows entrepreneurial leaders to develop their gifts in ways that would be impossible within the confining and restricting structures of most traditional denominations. These churches are strongly sensitive to the needs of their local communities, possessing a local grounding and knowledge which informs their strategies and agendas.

Perhaps the most celebrated recent example of a "community church" was established at Saddleback Valley, in Orange County, California, in 1980 by Rick Warren and his wife Kay, who had just graduated from Southwestern Baptist Theological Seminary in Texas. It aimed to reach out to those who did not traditionally attend church, in a way that was seeker-sensitive on the one hand, and theologically conservative on the other. The vision was to establish "a place where the hurting, the depressed, the frustrated, and the confused can find love, acceptance, help, hope, forgiveness, guidance, and encouragement." Warren's best-sellers The Purpose Driven Church (1995) and The Purpose Driven Life (2002) have had a significant impact on the reshaping of Protestant attitudes to creating community, evangelism, pastoral care and outreach - all unimpeded by any denominational apparatus.

So is this the future? There will be other big community churches which will develop distinctive ministries which work, which others will want to imitate. A major transformation of the religious life of the United States is under way, in which the mega-churches are, in effect, becoming the new dioceses, with large numbers of orbiting planets. They are more responsive to social changes, easier to manage and cheaper to run than traditional denominations.

Just as the great medieval monasteries planted smaller monasteries ("daughter houses") in outlying regions, resourced by the mother house until they were deemed strong enough to be self-sufficient, so the megachurches are in the process of spreading. The future of Protestant denominations in America may well be deeply shaped by this major new trend.

This phenomenon is set to increase, partly due to the seemingly inexorable dispersion of power from the centre to the periphery of traditional denominations, but more significantly on account of the enhanced responsiveness of such decentralized models to their specific (and changing) environments. As there is no centralized Protestant validating agency, equivalent to the Vatican, there is no enforceable means by which this phenomenon of innovation can be controlled.

The downside is obvious: there is no external accountability on the part of the pastor, who is often responsible only to congregation members or a core leadership group for the theological direction and pastoral conduct of the church. Yet the upside can hardly be ignored, especially when it seems to meet such a real need on the part of enthusiastic pastors and receptive congregations.

Always reforming

Now many Protestants would rightly wish to point out that much more can be said about ecclesiology than this. Anglicans, Lutherans and Mennonites, for example, would want to amplify (though in somewhat different ways) the minimalist Protestant ecclesiology outlined here. Such amplifications are, of course, to be welcomed. Yet they do not invalidate the fundamental point being made here - namely, that the 1520s Protestant vision of what is the "basic kit" of a true Christian church enables precisely the entrepreneurial and innovative approaches to church structures, growth and outreach that have proved so important to contemporary American Christian life. The same point could be made with reference to the "cell churches" that are playing a growing role in Protestantism in large Asian cities, such as Seoul and Manila.

Some will see this, for entirely understandable reasons, as a potentially negative development, leading to fragmentation, division and an erosion of the historical roots of Protestantism. I concede these dangers, to which others might easily be added, while believing that steps can be taken to deal with them. Yet one of the great historic roles of Protestantism has been to create conceptual space for these pioneering new models of "being church," and encourage highly entrepreneurial leaders to identify and rectify weaknesses in more traditional approaches to Christian ministry. One of the most distinctive and energizing features of Protestantism is its commitment to an agenda of self-examination and self-criticism, often summed up in the slogan ecclesia semper reformanda.

Yesterday's specific implementations of the movement's foundational ecclesiological vision, often shaped by the historical context of early modern Europe, cannot be determinative for global Protestantism today. The vision may stay the same; its actualization, however, must be constantly reviewed. As John Henry Newman pointed out many years ago, one of the greatest theological paradoxes is that we must change if we are to stay the same.

Today's experiments in ways of being church are to be seen and welcomed as an extension and continuation of the original Protestant vision, laid down at the time of the Reformation, rather than its contradiction or betrayal.

Alister McGrath is the Andreas Idreos Professor of Science and Religion at Oxford University. He is the author of The Intellectual Origins of the European Reformation and The Future of Christianity.