Major religious groups

The world's principal religions and spiritual traditions may be classified into a small number of major groups, though this is not a uniform practice. This theory began in the 18th century with the goal of recognizing the relative degrees of civility in different societies, but this concept of a ranking order has since fallen into disrepute in many contemporary cultures.
One way to define a major religion is by the number of current adherents. The population numbers by religion are computed by a combination of census reports and population surveys (in countries where religion data is not collected in census, for example the United States or France), but results can vary widely depending on the way questions are phrased, the definitions of religion used and the bias of the agencies or organizations conducting the survey. Informal or unorganized religions are especially difficult to count.
There is no consensus among researchers as to the best methodology for determining the religiosity profile of the world's population. A number of fundamental aspects are unresolved:
Religious traditions fall into super-groups in comparative religion, arranged by historical origin and mutual influence. Abrahamic religions originate in the Middle East, Indian religions in the Indian subcontinent (South Asia) and East Asian religions in East Asia. Another group with supra-regional influence are Afro-American religion, which have their origins in Central and West Africa.
Initially, Christians had a simple dichotomy of world beliefs: Christian civility versus foreign heresy or barbarity. In the 18th century, "heresy" was clarified to mean Judaism and Islam; along with paganism, this created a fourfold classification which spawned such works as John Toland's Nazarenus, or Jewish, Gentile, and Mahometan Christianity, which represented the three Abrahamic religions as different "nations" or sects within religion itself, the "true monotheism."
Daniel Defoe described the original definition as follows: "Religion is properly the Worship given to God, but 'tis also applied to the Worship of Idols and false Deities." At the turn of the 19th century, in between 1780 and 1810, the language dramatically changed: instead of "religion" being synonymous with spirituality, authors began using the plural, "religions", to refer to both Christianity and other forms of worship. Therefore, Hannah Adams's early encyclopedia, for example, had its name changed from An Alphabetical Compendium of the Various Sects... to A Dictionary of All Religions and Religious Denominations.
In 1838, the four-way division of Christianity, Judaism, Mahommedanism (archaic terminology for Islam) and paganism was multiplied considerably by Josiah Conder's Analytical and Comparative View of All Religions Now Extant among Mankind. Conder's work still adhered to the four-way classification, but in his eye for detail he puts together much historical work to create something resembling the modern Western image: he includes Druze, Yazidis, Mandaeans, and Elamites under a list of possibly monotheistic groups, and under the final category, of "polytheism and pantheism", he listed Zoroastrianism, "Vedas, Puranas, Tantras, Reformed sects" of India as well as "Brahminical idolatry", Buddhism, Jainism, Sikhism, Lamaism, "religion of China and Japan", and "illiterate superstitions" as others.
The modern meaning of the phrase "world religion", putting non-Christians at the same level as Christians, began with the 1893 Parliament of the World's Religions in Chicago. The Parliament spurred the creation of a dozen privately funded lectures with the intent of informing people of the diversity of religious experience: these lectures funded researchers such as William James, D. T. Suzuki, and Alan Watts, who greatly influenced the public conception of world religions.
In the latter half of the 20th century, the category of "world religion" fell into serious question, especially for drawing parallels between vastly different cultures, and thereby creating an arbitrary separation between the religious and the secular.
In Islam, the Quran mentions three categories: Muslims, the People of the Book, and idol worshipers.