Largest religions or belief systems by number of adherents

Christianity: 2.1 billion (Began: ca. 27 AD/CE)

• Roman Catholic: 1.05 billion
• Orthodox/Eastern Christian: 240 million
• African Indigenous Sects: 110 million
• Pentecostal: 105 million
• Reformed/Presbyterian/Congregational/United: 75 million
• Anglican/Episcopal: 73 million
• Baptist: 70 million
• Methodist: 70 million
• Lutheran: 64 million
• Jehovah's Witnesses: 15 million
• Latter-Day Saints: 12.5 million
• Adventists: 12 million
• Apostolic/New Apostolic: 10 million
• Stone-Campbell Restoration Movement: 5.4 million
• New Thought (Unity, Christian Science, etc.): 1.5 million
• Brethren (incl. Plymouth): 1.5 million
• Mennonite: 1.25 million
• Friends/Quakers: 300,000
  

Islam: 1.3 billion (Began: ca. 622 AD/CE)

• Sunnism: 940 million
• Shi'ism: 120 million
• Ahmadiyya: 10 million
• Druze: 450,000

Secular/irreligious/agnostic/atheist/antitheistic/antireligious: 1.1 billion

• Category includes a wide range of beliefs, without specifically adhering to a religion. The category also includes humanism, deism, pantheism, freethought, and Juche.
  

Hinduism: 900 million (Began: approximately 1500 BC/BCE or 15th century BC/BCE however some aspects of it trace its history to 2600 BC/BCE or 26th century BC/BCE)

• Vaishnavism: 580 million
• Shaivism: 220 million
• Neo-Hindus and Reform Hindus: 22 million
• Veerashaivas/Lingayats: 10 million
  

Chinese folk religion: 394 million

• Not a single organized religion, includes elements of Taoism, Confucianism, Buddhism and traditional nonscriptural religious observance (also called "Chinese traditional religion").

Buddhism: 376 million (Began: 6th century BC/BCE)

• Mahayana: 185 million
• Theravada: 124 million
• Lamaism (Vajrayana/Tibetan): 20 million

Primal indigenous: 300 million

• Not a single organized religion, includes a wide range of primarily Asian traditional or tribal religions, including Shamanism and Paganism.

African traditional and diasporic: 100 million

• Not a single organized religion, this includes several traditional African beliefs and philosophies such as those of the Yoruba, Ewe (vodun) and the Bakongo. These three religious traditions (especially that of the Yoruba) have been very influential to the diasporic beliefs of the Americas such as condomble, santeria and voodoo. The religious capital of the Yoruba religion is at Ile Ife.

Sikhism: 23 million (Began: 1500s AD/CE)
Spiritism: 15 million (Began: mid-19th century AD/CE)

• Not a single organized religion, includes a variety of beliefs including some forms of Umbanda.

Judaism: 14 million (Began: 13th century BC/BCE)

• Conservative: 4.5 million
• Unaffiliated and Secular: 4.5 million
• Reform: 3.75 million
• Orthodox: 2 million
• Reconstructionist: 150,000

Bahá'í Faith: 7 million (Began: 19th century AD/CE)
Jainism: 4.2 million (Began: 6th century BC/BCE)

• Svetambara: 4 million
• Sthanakvasi: 750,000
• Digambara: 155,000

Shinto: 4 million (Began: 300 BC/BCE)

• This number states the number of actual self-identifying practicing primary followers of Shinto; if everyone were included who is considered Shinto by some people due to ethnic or historical categorizations, the number would be considerably higher — as high as 100 million.

Cao Dai: 4 million (Began: 1926 AD/CE)
Zoroastrianism: 2.6 million (Began: ca. 6th century BC/BCE)

• Parsis: 110,000
• Gabars: 20,000

Falun Gong: 2.1 million* (Began: 1992 AD/CE)

• Not necessarily considered a religion by adherents or outside observers. No membership or rosters, thus the actual figure of practitioners is impossible to confirm.

Tenrikyo: 2 million (Began: 1838 AD/CE)
Neopaganism: 1 million (Began: 20th century AD/CE)

• A blanket term for several religions like Wicca, Asatru, Neo-druidism, and many reconstructionist religions

Unitarian Universalism: 800,000 (Began: 1961 AD/CE)
Rastafari: 600,000 (Began: early 1930s AD/CE)
Scientology: 500,000 (Began: 1952 AD/CE)

Src wikipedia, adherents.com