Orthodox Christianity: The oldest established eastern form of Christianity, the Holy Orthodox Church, has a ceremonial head in the Bishop of Constantinople (Istanbul), also known as a Patriarch, but its various regional forms (e.g., Greek Orthodox, Russian Orthodox, Serbian Orthodox, Ukrainian Orthodox) are autocephalous (independent of Constantinople's authority, and have their own Patriarchs). Orthodox churches are highly nationalist and ethnic. The Orthodox Christian faith shares many theological tenets with the Roman Catholic Church, but diverges on some key premises and does not recognize the governing authority of the Pope.
Protestant Christianity: Protestant Christianity originated in the 16th century as an attempt to reform Roman Catholicism's practices, dogma, and theology. It encompasses several forms or denominations which are extremely varied in structure, beliefs, relationship to state, clergy, and governance. Many protestant theologies emphasize the primary role of scripture in their faith, advocating individual interpretation of Christian texts without the mediation of a final religious authority such as the Roman Pope. The oldest Protestant Christianities include Lutheranism, Calvinism (Presbyterians), and Anglican Christianity (Episcopalians), which have established liturgies, governing structure, and formal clergy. Other variants on Protestant Christianity, including Pentecostal movements and independent churches, may lack one or more of these elements, and their leadership and beliefs are individualized and dynamic.
Hinduism - Originating in the Vedic civilization of India (second and first millennium B.C.), Hinduism is an extremely diverse set of beliefs and practices with no single founder or religious authority. Hinduism has many scriptures; the Vedas, the Upanishads, and the Bhagavad-Gita are among some of the most important. Hindus may worship one or many deities, usually with prayer rituals within their own home. The most common figures of devotion are the gods Vishnu, Shiva, and a mother goddess, Devi. Most Hindus believe the soul, or atman, is eternal, and goes through a cycle of birth, death, and rebirth (samsara) determined by one's positive or negative karma, or the consequences of one's actions. The goal of religious life is to learn to act so as to finally achieve liberation (moksha) of one's soul, escaping the rebirth cycle.
Islam - The third of the monotheistic Abrahamic faiths, Islam originated with the teachings of Muhammad in the 7th century. Muslims believe Muhammad is the final of all religious prophets (beginning with Abraham) and that the Qu'ran, which is the Islamic scripture, was revealed to him by God. Islam derives from the word submission, and obedience to God is a primary theme in this religion. In order to live an Islamic life, believers must follow the five pillars, or tenets, of Islam, which are the testimony of faith (shahada), daily prayer (salah), giving alms (zakah), fasting during Ramadan (sawm), and the pilgrimage to Mecca (hajj).
Basic Groupings
The two primary branches of Islam are Sunni and Shia, which split from each other over a religio-political leadership dispute about the rightful successor to Muhammad. The Shia believe Muhammad's cousin and son-in-law, Ali, was the only divinely ordained Imam (religious leader), while the Sunni maintain the first three caliphs after Muhammad were also legitimate authorities. In modern Islam, Sunnis and Shia continue to have different views of acceptable schools of Islamic jurisprudence, and who is a proper Islamic religious authority. Islam also has an active mystical branch, Sufism, with various Sunni and Shia subsets.
Sunni Islam accounts for over 75% of the world's Muslim population. It recognizes the Abu Bakr as the first caliph after Muhammad. Sunni has four schools of Islamic doctrine and law - Hanafi, Maliki, Shafi'i, and Hanbali - which uniquely interpret the Hadith, or recorded oral traditions of Muhammad. A Sunni Muslim may elect to follow any one of these schools, as all are considered equally valid.
Shia Islam represents 10-20% of Muslims worldwide, and its distinguishing feature is its reverence for Ali as an infallible, divinely inspired leader, and as the first Imam of the Muslim community after Muhammad. A majority of Shia are known as "Twelvers," because they believe that the 11 familial successor imams after Muhammad culminate in a 12th Imam (al-Mahdi) who is hidden in the world and will reappear at its end to redeem the righteous.
Variants
Ismaili faith: A sect of Shia Islam, its adherents are also known as "Seveners," because they believe that the rightful seventh Imam in Islamic leadership was Isma'il, the elder son of Imam Jafar al-Sadiq. Ismaili tradition awaits the return of the seventh Imam as the Mahdi, or Islamic messianic figure. Ismailis are located in various parts of the world, particularly South Asia and the Levant.