Wilberforce and his circle believed that if a sinful system existed and its sinfulness implicated them as well as others, they were under an obligation to end it. There is no simple gulf between personal and public morality; and Christian morality is not about "keeping yourself unspotted from the world" in any sense that implies withdrawing or ignoring public wrongs.

But if the state enacts or perpetuates in the corporate life of the nation what is directly contrary to the Christian understanding of God's purpose, then Christian activism in respect of changing the law is justified, primarily when the state is responsible for - so to speak - compromising the morality of all its citizens.

This is at the heart of what Wilberforce was concerned about. He is not campaigning for the state to impose a personal morality, and would probably have agreed that such a policy would take away the essential aspect of personal liberty in choices about one's own life. But he is campaigning for a moral state - that is, for a state that does not compromise its citizens, and that recognises wider considerations than those of immediate profit and security.

This makes sense, though, only if it is possible to convince those who run things in the public sphere that there are human values and ethical norms to which an entire society is answerable. In our relativist climate, this is very difficult. What tends to happen is that nothing much is left as a substantive moral basis for public life except a poorly defined principle of tolerance or avoidance of mutual harm. The idea that you can give substance to a common social ethic, something to which society as a whole can be held accountable, is unfashionable and unwelcome. Even from the point of view of many who have no religious commitment, there is a recognition that this is a thin diet.

But the problem is deeper still. Without a notional standard of human excellence and human flourishing, the definition of what is good for people is always going to be vulnerable to what happens to suit a dominant interest group.

Take away that sense, and it is a great deal harder to think about political life as a vocation. It is significant that in the past few years one of the most widely supported political campaigns was the Jubilee 2000 movement for the remission of unpayable Third World debt; outside - but not in opposition to - the parliamentary process, many hundreds of thousands lobbied for a change in what the administration regarded as legally possible. It seemed to be assumed that parliamentary campaigning would not deliver the same effective results.

I'm not implying that this perception is fair or that our culture has simply swung against old-style parliamentary campaigning for good, only that we are in a disturbing position if, on major issues of public morality, people expect to make a change outside rather than within the electoral system.

I believe that it is possible for a state to have a moral basis without thereby becoming confessional or theocratic. It involves a state being ready to recognise its own history; to say that its horizons and assumptions are indeed grounded in a set of particular beliefs, and to embody in its political practice ways of allowing those foundational commitments to be heard in public debate.

The establishment of the Church of England, as it has evolved in the past century or so, has turned out to be such a mechanism - a rather awkward one. But if the church were to be disestablished, the question would still be there in an acute form: how does the state properly expose itself to argument about its collective moral status?

It is a point that has emerged in one of the most unusual of contemporary societies, China. The collapse of straightforward Marxist philosophy as the moral basis of the state has left a disturbing vacuum; and the state has had to rethink its attitude to religion in surprising ways.

What is needed in Britain is a set of conventions that will allow government to hear voices unconstrained by electoral anxiety and narrow considerations of practical profitability.

It is important that in our current debates about the upper house of parliament we take seriously the role of such a house in offering channels of independent moral comment. Whatever model we devise for the upper house, it is vital to ensure that it is not simply swallowed up in an electoral system that could remove this degree of moral independence.

And I make no apology for saying that the nature and extent of religious representation in the upper house - a bigger issue than the number of Anglican bishops holding seats there - is not a marginal question at all in the light of this discussion.

Wilberforce believed politics was a vocation because he saw politics as always opening out beyond itself. Our democracy is very different now from what it was in 1806, but some of the dangers are much the same.

---This is an extract from a lecture to be given on Tuesday in Hull by Rowan Williams, Archbishop of Canterbury