What is "liberal" Christianity? Is it the attempt to do Christianity in an honest, modern way; or is it an attempt to dodge the hard bits of this faith? I have spent quite a large proportion of my life thinking this through, wondering whether liberal Christianity can be authentically Christian, or whether it gravitates towards a soggy compromise with secular humanism.

Well, eureka, I think have found the answer. The conclusion I have come to is that liberal Christianity has two meanings: there are two traditions here. They are deeply intertwined, but they must be pulled apart – for one tradition infects and corrodes the other. Only once this separation is made can an authentic liberal Christianity be affirmed.

One sort of liberal Christianity edges away from supernatural belief, and church ritual: it presents Jesus as a great moral teacher, the first humanist, through whose example we can learn to mend our world. It assumes a basic harmony between Christianity and the rational Enlightenment.

The other sort of liberal Christianity affirms political liberalism – the ideal of a state that rejects theocracy and protects people's liberties. But it does not seek to reform Christianity in a rational-humanist direction: it understands that such "reform" undermines this religion, falsifies it.

Very simply, the latter sort of liberal Christianity is the only authentic version; it must be rescued from the deathly embrace of the former sort. Only thus can liberal Christianity be renewed.

As a callow youth I was drawn to the sort of liberal theology that claims to move beyond the irrational aspects of religion. But soon I began to wonder whether such liberalism led to Christian-tinged humanism, indistinguishable from agnosticism. I required a stronger form of Christianity – religious socialism seemed to fit the bill for a while. But there were strong humanist assumptions here too, I gradually admitted. I studied theology, and learned that cutting-edge thought was strongly "post-liberal"; it sought to purge theology of Enlightenment corruption, and restore its autonomy. I largely agreed with this agenda, but retained a nagging sense that such theology over-reacted against liberalism.

After 9/11, I found myself strongly drawn to one aspect of liberalism: the separation of church and state. In a new era of religious tension, it seemed necessary for the liberal state to clarify its treatment of religion in this way. The establishment of the C of E, something I'd taken for granted all my life, began to seem acutely problematic. So did the church's role in state education.

So I became more decidedly liberal, in one sense, but retained a deep ambivalence about liberal theology, knowing how strongly it gravitated to humanism. I wanted to criticise the institutional conservatism of the churches; but didn't such criticism lead to post-Christian humanism? How could the right sort of liberalism be distinguished from the wrong sort?

Ad fontes! Go to the origins. I made a fresh attempt to understand the historical source of the problem. I found that the Reformation gradually gave rise to two forms of liberal Christianity. One of these was deeply involved in the first phase of political liberalism, in the mid-17th century. Its clearest theorist was the poet John Milton. He said that the Protestant Reformation, launched a century earlier, had now entered a new phase. God willed a new sort of state, with no official church enforcing religious unity. Instead, the state should protect people's freedom to believe and worship as they wanted (as long as they did not threaten the new political order).

He criticised any authoritarian church, established or not, that tried to impose moral and religious rules on people (he pointed out that St Paul had attacked such rules). This is the liberal Christian tradition that I affirm. It is a religious vision that entails a political vision, of the post-absolutist state, in which the ideal of liberty unites people.

But something else happened in the 17th century. Protestantism gradually absorbed rationalist assumptions about the need to reform Christianity away from both ritual and supernatural belief. This was a disaster. For authentic Christianity cannot dispense with faith, nor with ritual expression. If it cuts itself off from the basic, rationally unjustifiable practices of worship (prayer-speech, communal cultic action), it commits suicide. One need only recall that this is a religion that makes regular use of (what might be called) fake blood – that involves the drinking of fake blood!

Liberal Christianity became dominated by this rational-humanist reformism. A nuanced liberal Christianity – one that affirmed political liberty, but also understood that authentic Christianity must be rooted in faith and ritual practice – failed to emerge. Even the best Protestant theologians (Kierkegaard, Karl Barth) contributed to the problem, by treating "liberal theology" as a unity.

Only connect. We must strive to reaffirm the best of liberal Christianity, without recycling the worst. This tradition can, and must, be reinvented. In the second article, next Sunday, I'll suggest what the way forward might look like.