Last winter I finally got to Ravenna, and to the Church of San Vitale. There, in mosaics, Justinian and Theodora, with their courtiers and tame priests, face one another across the chancel and in the apse a beardless Christ appears in a green meadow surrounded by flowers, lambs and fountains.

I wondered how, after this, things could have gone so wrong. Christianity went from the flowering meadow and Byzantine court to the tortured crucifixions of the Northern Renaissance. Exuberance and wonder gave way to a crabbed obsession with sin and Christians turned inward to fret about the health of their souls.

There is, however, a tradition that suggests this enterprise is doomed. "He that findeth his life shall lose it: and he that loseth his life for my sake shall find it." Wrapped up in our selves and preoccupied with working out our salvation, we defeat our purpose.

This is reminiscent of the paradox of hedonism – the recognition that if we aim for happiness it will elude us. John Stuart Mill observes: "Those only are happy who have their minds fixed on some object other than their own happiness … Aiming thus at something else, they find happiness along the way." So long as we are all about our ourselves, we will never be satisfied. If our goal is our own salvation, we will not achieve it.

The better class of evangelical preachers these days – those who aim to attract a more upscale clientele to their suburban megachurches – don't say much about salvation. Like the secular self-help gurus they promise personal growth, "recovery" and other this-worldly benefits. But the message is the same: it's all about you. The church provides you with uplift, recipes for successful living, community, consolation, and encouragement – to meet your needs and satisfy your wants.

The rhetoric and politics of evangelicals and self-help gurus are different but the essential message is the same: it is the doctrine of salvation by faith according to the canonical born-again scenario. You are seized by the conviction that your life is profoundly unsatisfactory in some global way that eludes further analysis: you are sinful, neurotic, stressed, addicted, co-dependent – insufficiently happy. You assemble a customised mix of the beliefs that "work" for you: your personal faith. Through it you achieve salvation, healing, and personal growth, the start of an endless spiritual journey to further self-improvement. You, and other Americans whose only hobbies are themselves, support legions of pastors, gurus, therapists and motivational speakers.

It is easy to see why most people are contemptuous of this amalgam of credulity, sentimentality and narcissism, which in its evangelical Christian form is tied up with myths about the age of the earth and origin of species, sexual taboos and a conservative political agenda. With this as the public face of religion it's not surprising that in the US, as in Europe, Christianity is collapsing.

That is a shame because if it collapses everything essential to it and worthwhile, which is now merely obscure, will become inaccessible. Christian theology, metaphysical doctrines about the existence and nature of God that I believe to be true, will become curiosities, like the teachings of second-string neo-Platonists. Service books will languish in archives, for study by antiquarians. The better churches will be preserved as museums; mediocre ones will be gutted and refurbished as restaurants, condos and office space.

In Europe religious belief is already anomalous: San Vitale is a museum. In the US, Christianity has been absorbed into a syncretic mishmash of self-help programmes and therapies, new age products and scraps of eastern religions. There are cults for every taste and circumstance. Maybe some coherent religious system will surface in the way that Christianity emerged from the soup of cults and mystery religions in the Hellenistic world. Maybe Christianity will re-emerge. Maybe.