Religion has many different definitions and people often philosophize over the many definitions on a daily basis. While there are quite a few major religions there is only one most popular religion in the world.
Christianity, Islam, Hinduism and Buddhism are the most popular world religions today. Approximately 1 billion people adhere to irreligious beliefs which include humanism, atheism and agnosticism, making this category of beliefs more popular than Hinduism but less popular than Islam.
Christianity is the most popular religion in the world with about 2.1 billion adherents as of 2005. The most popular religion only comprised around 33% of the total world population. Christianity considered as such a group includes differing denominations such as Roman Catholics, Southern Baptists, and Mormons.
In ranking religious denominations, the Roman Catholic Church is the most popular denomination within Christianity.