The world's largest religion is Christianity, making up about 33% of the world population according to a 2005 survey by the Encyclopædia Britannica. That is approximately two billion people. Other large religions include Islam (20%), Hinduism (13%), Chinese folk religion (6.3%) and Buddhism (5.9%). Indigenous religions make up 4% of the world's population, and atheism about 14%.
Of two billion Christians, about half are represented by the Roman Catholic Church, led by the Pope, with the rest being members of the Eastern Orthodox Church or various Protestant sects. The Christian Church suffered several schisms throughout its history, most importantly the East-West Schism (1054) and the Protestant Reformation (16th century).
Christianity, which began as a Jewish sect, has been the world's largest religion for about 1600 years, since on 27 February 380 when Emperor Theodosius I made Christianity the official religion of the Roman Empire, ending hundreds of years of Roman mythology as state religion. Before that, the world's largest religion was likely Hinduism, which is so old that its beginnings are poorly understood. Christianity received another major boost in the 980s, when Vladimir the Great was baptized and spread Christianity throughout Russia. If Christianity was not the world's largest religion before then, then it surely became the largest at that time.
Christianity received further gains in the 15th and 16th centuries, when missionaries, especially the Spanish and Portuguese, aggressively spread the religion throughout Africa and the Americas. Today, South America remains the most exclusively Christian continent on the planet, with more than 90% of its residents claiming the Christian faith, almost entirely Roman Catholicism. In recent years, Christianity, while remaining the world's largest religion, has been experiencing a consistent decline in industrialized regions like the United States and Europe, while experiencing gains in developing regions like Africa and South America. The declines are particularly pronounced in countries like Denmark and Sweden, where only 2% of the population regularly attends church.
Although Christianity was followed by about a third of the world's population throughout the 20th century, the religion also faced its greatest challenges throughout this period, with the rise of communism (which makes atheism the state religion) and skepticism. However, with the collapse of the Soviet Union, many former atheists are starting to express their Christian faith again, and in Russia there is a close relationship between the Russian Orthodox Church and the government.