The journey is that of Ed Stourton, the BBC newsreader who has taken the skills he learned as a foreign correspondent in Washington and Paris and brought them to bear on something rather nearer home - his own faith as a Roman Catholic. This week saw the conclusion of his BBC2 blockbuster series which he called Absolute Truth in deference to that Church's claims to be its repository. And if the result has irritated some conservatives, that is merely a measure of how far Stourton has travelled since his days at the Benedictine public school, Ampleforth, where he was known as a straightforward traditionalist.
The sausage goes back before his Ampleforth days. At the age of 12, he found the half-eaten object in the holy-water stoop at the chapel of his Catholic prep school in Sussex. This was not a harmless piece of schoolboy naughtiness, he felt at the time, but an act of rebellion and sacrilege. "The perpetrator had chosen a symbol of all that was most precious and sacred to deliver a calculated insult, an iconoclastic rejection of the Natural Order as we understood it," he writes in the book which accompanies the series. He and his fellow prefects hunted down the wretched rebel.
Looking back, it is a perfect example of how time and place determine the character of faith. His Catholicism then was as unchanging and settled as the hierarchical boarding school in which he found himself and as the countryside in which the school was set. It was that world of English Catholicism where snobbery comes dressed as sanctity.
"I suppose when I began making the programmes, I was a mild conservative," he told me. "But by the end I had found the arguments of the progressives to be persuasive."
It was a journey, he found, which others had made before him. One of the most moving sequences in the series was the reconstruction of the death of Archbishop Oscar Romero, who was murdered at the altar in El Salvador. Romero, too, had begun as a conservative whose appointment was endorsed by right-wing politicians in his own country and traditionalists in Rome. But the reality of his contact with the country's poor changed him into so potent an advocate of social justice that the army's death squads felt he had to be eliminated.
It was not the only transformation on the ground. Stourton went to a Zambia shanty town where he met Sister Leonia, a Polish nun who works there as a nurse. Once she was loyal to the Church's ban on artificial contraceptives. But contact with her patients - 93 per cent of whom were HIV positive - changed her mind. "I am also a human being and I am touching the problem," she said. "It's my opinion and I hope God will forgive me for it."
The fault-line which Stourton explores runs between the notions of the Church which held sway before and after the revolutionary Second Vatican Council began in 1965. It is a debate, he says, "which is, if anything, more alive now than it has ever been".
The Council was launched by Pope John XXIII in an attempt to reconcile Catholicism with the philosophical inheritance of the Enlightenment and drag the church, if not into the 20th century, then at least into the 18th. The Council overturned the old notion of a Church turned inward on its sacramental life and the preservation of ancient formularies. In its place, it put an outward-focused Church which interacted with the world and sought evidence of divine activity in the "the signs of the times" in contemporary life. But the battle between the two worldviews continues, with the wave of new thinking and practices unleashed by the Council now being reined in by Pope John Paul II, who clearly thinks things are spinning out of control of his centralised, authoritarian curia.
The present pope has not gone as far as an earlier one, Sixtus, who once ordered that a wit have his hands cut off and his tongue slit. But Stourton did visit a man who has suffered the modern equivalent.
Father Tissa Balasuriya is a Sri Lankan priest noted for his dialogue with Hinduism, something on which the Pope is distinctly ambivalent. The Asian priest's book, Mary and Human Liberation, turns the usual image of the Blessed Virgin inside out; instead of a humble, compliant, otherworldly figure, he depicts her as a feminist icon, political activist and the first woman priest, standing, as she did, at the foot of the cross.
When Balasuriya decided to popularise the book for ordinary believers, by means of a video, the pre- and post-conciliar visions of Catholicism clashed and Rome excommunicated him (a decision since rescinded after a worldwide outcry). To Stourton, the incident highlighted not just the renewed intolerance of free expression within the church, but also the extent to which its paradigms were forged in a European culture, outside which the vast majority of the world's Catholics now live.
"The heart of the Church is now elsewhere, and its eternal truths must be expressed in different ways," Stourton concludes. "Catholicism may be about sex in Europe but in Latin America it is about poverty and in Africa and Asia it is about identity." Rome has to find new ways of giving human experience due weight next to the Church's understanding of revealed truth. Only then will it discover whether the faith has "the answers that mankind needs as the third millennium of the Christian era begins."
`Absolute Truth' by Edward Stourton is published by Viking, pounds 20Reuse content