Short answers, pulled from the story.
Christianity is the world's largest religious group, with 2.3 billion followers, representing 28.8 percent of the global population according to Pew Research Centre's 2025 report covering changes from 2010 to 2020. Islam is the second largest, with 2 billion adherents at 25.6 percent.
The classification of major religious groups began in the 18th century, initially as a way to rank societies by their perceived degrees of civility. By the early 19th century, between 1780 and 1810, scholars began using the plural "religions" to refer to both Christianity and other forms of worship.
Major traditions are grouped by geographic and historical origin: Abrahamic religions (Judaism, Christianity, Islam, Bahai Faith) from the Middle East; Indian religions (Hinduism, Buddhism, Jainism, Sikhism) from the Indian subcontinent; East Asian religions (Taoism, Confucianism, Shinto); African and indigenous ethnic religions; and Modern Paganism.
According to Pew Research Centre's 2025 report, 1.9 billion people, or 24.2 percent of the global population, are religiously unaffiliated. This makes the unaffiliated the third largest group worldwide, behind Christians and Muslims.
The 1893 Parliament of the World's Religions, held in Chicago, established the modern meaning of the phrase "world religion" by placing non-Christian traditions at the same level as Christianity. The Parliament led to a dozen privately funded lecture series that supported researchers including William James, D. T. Suzuki, and Alan Watts.
Religious population figures are computed from a combination of census reports and population surveys; countries such as the United States and France do not collect religion data in their national censuses at all. Results vary depending on how survey questions are phrased, which definition of religion is applied, and whether researchers count active practitioners, self-identified adherents, or those born into a tradition.