The biggest religion in the world is Christianity at 33% of the world’s population. There was a great schism in the 1050′s between the East and the West. The Eastern Orthodox Church does not recognize the supreme right of the Pope, whereas the Western Catholics, now known as Roman Catholic Church recognize the Pope as the supreme authority. Another difference is that the Eastern Orthodox Church conducts its services in local languages and Roman Catholics stick to Latin. And of the geographical separation; Roman Catholicism is considered "Western” and the Eastern Orthodox encompasses Eastern European countries and Russia. Obviously, there are many subdivisions within these two categories, but the photo to the left will give you an idea of the evolution (ironic word choice) of how the different religions within Christianity are formed.
The next biggest religion is Islam, started in the 7th century. The holy book is the Qur’an that was communicated to Muhammad directly from God.
Interestingly, Muslims don’t believe that Muhammad founded a new religion, but rather that he restored the original ideals put forth by Abraham, Moses, Jesus and others. They believe that Christians have distorted the original message. Like Christianity, Muslims developed a major rift somewhere in the 7th century and were divided between Sunni and Shi’a. The major philosophical difference between the two is that Shi’a recognize an Imam as the interpreter and ruler of laws and tradition. They hold close the idea of the martyrdom of Ali and Hussein, the last descendants of the prophet Muhammad. Sunni’s believe that Shi’as don’t adhere strictly to the 5 Pillars of Islam and focus on the martyrdoms of Ali and Hussein too much. Shi’as form only 15% of all Muslims, but dominate many Middle Eastern countries; Iran, Yemen, Azerbaijan, Bahrain and Iraq. Hezbollah is run by Shi’as and was responsible for. Sunnis closely follow the path of Muhammad’s life as this is their main tenet of faith. The diagram to the left breaks down Islam into different subcategories.
The third biggest religion is Hinduism at 13% of the world and non religious at 12% of the world.