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, Professor MacCulloch tells how confession was invented by monks on 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.

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.

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.

In the second episode, Nathan develops a rehearsal for Angela, a woman considering motherhood. Nathan hires child actors to simulate adopting and caring for a baby and sets her up in a rented farmhouse in rural Oregon. Due to Oregon child protection laws, Nathan's team must covertly switch out the baby every four hours and replace it with a robot baby at night. Seeking a simulated husband, Angela dates Robbin, a numerology-obsessed man who wants to have sex with Angela despite her devout Christian beliefs against premarital sex. When Robbin quits the project due to the robot baby's incessant crying, Nathan inserts himself into the experiment as Angela's non-romantic co-parent.

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?

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.