Diarmaid MacCulloch's own life story makes him a symbol of a distinctive feature about Western Christianity - scepticism, a tendency to doubt which has transformed both Western culture and Christianity. In the final programme in the series he asks where that change came from. He challenges the simplistic notion that faith in Christianity has steadily ebbed away before the relentless advance of science, reason and progress and shows instead how the tide of faith perversely flows back in. Despite the attacks of Newton, Voltaire, the French Revolutionaries and Darwin, Christianity has shown a remarkable resilience. The greatest damage to Christianity was actually inflicted to its moral credibility by the two great wars of the 20th century and by its entanglement with fascism and Nazism. And yet it is during crisis that the Church has rediscovered deep and enduring truths about itself, which may even be a clue to its future.

When he was a small boy, Diarmaid MacCulloch's parents used to drive him round historic churches. Little did they know that they had created a monster, with the history of the Christian Church becoming his life's work. In the first of a six-part series sweeping across four continents, Professor MacCulloch goes in search of Christianity's forgotten origins. He overturns the familiar story that it all began when the apostle Paul took Christianity from Jerusalem to Rome. Instead, he shows that the true origins of Christianity lie further east, and that at one point it was poised to triumph in Asia, maybe even in China. The headquarters of Christianity may well have been Baghdad not Rome, and if that had happened then western Christianity would have been very different.

Today, Eastern Orthodox Christianity flourishes in the Balkans and Russia, with over 150 million members worldwide. It is unlike Catholicism or Protestantism - worship is carefully choreographed, icons pull the faithful into a mystical union with Christ, and everywhere there is a symbol of a fierce-looking bird, the double-headed eagle. What story is this ancient drama trying to tell us? In the third part of his journey into the history of Christianity, Professor Diarmaid MacCulloch charts Orthodoxy's extraordinary fight for survival. After its glory days in the eastern Roman Empire, it stood right in the path of Muslim expansion, suffered betrayal by crusading Catholics, was seized by the Russian tsars and faced near-extinction under Soviet communism. MacCulloch visits the greatest collection of early icons in the Sinai desert, a surviving relic of the iconoclastic crisis in Istanbul and Ivan the Terrible's cathedral in Moscow to discover the secret of Orthodoxy's endurance.

The Amish today are peaceable folk, but five centuries ago their ancestors were seen as some of the most dangerous people in Europe. They were radicals - Protestants - who tore apart the Catholic Church. In the fourth part of the series, Diarmaid MacCulloch makes sense of the Reformation, and of how a faith based on obedience and authority gave birth to one based on individual conscience. He shows how Martin Luther wrote hymns to teach people the message of the Bible, and how a tasty sausage became the rallying cry for Swiss Reformer Ulrich Zwingli to tear down statues of saints, allow married clergy and deny that communion bread and wine were the body and blood of Christ. 'Jesus ascended into heaven', declared Zwingli. 'He's sitting at the right hand of the Father, not on a table here in Zurich.'.

Diarmaid MacCulloch traces the growth of an exuberant expression of faith that has spread across the globe - Evangelical Protestantism. Today, it is associated with conservative politics, but the whole story is distinctly more unexpected. It is easily forgotten that the Evangelical explosion has been driven by a concern for social justice and the claim that one could stand in a direct emotional relationship with God. It allowed the Protestant faith to burst its boundaries from its homeland in Europe. In America, its preachers marketed Christianity with all the flair and swashbuckling enterprise of American commerce. In Africa, it converted much of the continent by adapting to local traditions, and now it is expanding into Asia. But is Korean Pentecostalism and its message of prosperity in the here and now an adaptation too far?

Professor Diarmaid MacCulloch's grandfather was a devout pillar of the local Anglican church and felt that any dabbling in Catholicism was liable to pollute the English way of life. But now his grandfather isn't around to stop him exploring the extraordinary and unpredictable rise of the Roman Catholic Church. Over one billion Christians look to Rome, more than half of all Christians on the planet. But how did a small Jewish sect from the backwoods of 1st century Palestine, which preached humility and the virtue of poverty, become the established religion of western Europe - wealthy, powerful and expecting unfailing obedience from the faithful? Amongst the surprising revelations, MacCulloch tells how confession was invented by monks in a remote island off the coast of Ireland, and how the Crusades gave Britain the university system. Above all, it is a story of what can be achieved when you have friends in high places.