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.

• Diarmaid MacCulloch