The worl's principal religions and spiritual traditions may be classified into a small number of major groups or world religions. According to the 2005 survey of Encyclopædia Britannica, the vast majority of religious and spiritual adherents follow Christianity (33.06% of world population), Islam (20.28%), Hinduism (13.33%), Chinese folk religion (6.27%) or Buddhism (5.87%). The irreligious and atheists are 14.27% and 3.97% follow indigenous tribal religions.