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.

David Attenborough concludes his epic history of the evolution of flight with an exploration of the highly advanced fliers that dominate our skies today – the extraordinarily diverse skills of the birds, and the sonar-guided precision of the bats. He encounters some of our planet’s most remarkable fliers – from peregrine falcons dive-bombing starlings over Rome, to hummingbirds hovering in the cloud forests of Ecuador. In a spectacular finale, he visits Gomantong cave in Borneo to witness the mass exodus of a million bats.

Moving on to the Late Eocene period 36 million years ago and mammals have prospered and are now the largest creatures on land and sea. This is an era of animals like andrewsarchus, the biggest mammal carnivore ever to walk on land, and the brontotheres, small-brained herbivores. It is in the sea, however, that the most monstrous mammals of all can be found. We follow the fate of a female basilosaurus, a huge serpent-like early whale, but nothing like the gentle filter feeding whales of the 21st century. Four times the length of the great white shark, with jaws to match, she is every inch a killer.

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?