The Orthodox and Roman Catholics went their separate ways in 1054, a historic event known as Great Schism. During the Middle Ages, the Orthodox Church viewed itself as the protector of Christianity, and the heir to Rome. Today, it is the largest Christian denomination in Russia, with an estimated 150 million adherents, active or passive, worldwide. By comparison, there are around 780,000 Catholics in Russia, according to the World Christian Database. Still, the Orthodox Church fears that a greater presence for Catholics in Russia will result in Catholics converting (i.e. poaching) people the Orthodox claims as their own. The growth of Protestant churches, as well as evangelical and Pentecostal denominations, has also been worrisome to the Russian church, and prompted restrictions and regulations in recent years.

Claudio Peri/Pool/AP
Pope Francis (r.) and Russian President Vladimir Putin (l.) share a private audience at the Vatican, Nov. 25. Putin didn't invite Francis for a return visit.