Merriam Webster defines religion as "service and worship of God or the supernatural" or "commitment or devotion to religious faith or observe," and also as "a personal set or institutionalized system of religious attitudes, beliefs and practices." In combination, these definitions describe what is commonly meant by religion, although scholars have been striving for centuries to find a definition of religion that encompasses its varied aspects and practices.
Religion indeed includes cultural and belief systems and worldviews relating to spirituality and often moral values. Many religions attempt to explain the meaning of life, the origin of existence, and what our relationship is to the divine. Most include traditions, teaching and or sacred texts, and religious laws. Most also have a definition for what constitutes adherence to the faith.
Comparative religion is the grouping of religions according to historical origin, geographic origin, and similarities or mutual influence. Comparative religion groups the worlds' religions into the following categories: Abrahamic religions, Indian religions, Far Eastern Religions, African diasporic religions, Iranian religions and indigenous ethnic religions.
Abrahamic religions all reference Abraham as a patriarch, and include Christianity, Islam, Judaism and Baha'i Faith. Mandaeism, Rastafari, Samaritanism, and Druzism are among the smaller Abrahamic religions. Over 3 billion people are believed to be adherents to the Abrahamic religions. Indian religions tend to share key tenets, such as dharma and karma, and include Hinduism, Buddhism, Sikhism and Jainism. The East Asian religions include the Chinese concept of Tao ("Do" in Korean or Japanese) and include Confucianism and Taoism. African diasporic religions have their roots in the religions of Central and West Africa. Iranian religions include Zoroastrianism and overlaps with Abrahamic religions in the case of Sufism and Baha'I Faith. Indigenous ethnic religions include the traditions of Native Americans and Australian Aboriginals, Asian Shamanism and Shinto.
The world's biggest religions in terms of adherents are Christianity, Islam, Hinduism, Judaism and Buddhism. Of these, Hinduism (formed between 4000 and 2500 B.C.E.) is the oldest and Islam, founded in 622 C.E. by Muhammad the Prophet) is the youngest. Christianity (formed around 30 C.E.) and Islam have their roots in Judaism, formed around 2000 B.C.E.