Christianity
Christianity comprises a group of religious traditions originating with Jesus Christ that generally assert that Jesus is the son of God, and the messiah: the sole savior of all humanity. That is, Jesus redeemed mankind from their sins (i.e. faults, misdeeds, rebellion against God), reconciling mankind to God so that man can live eternally with God in a state of never-ending happiness. The Christian religions, taken together, form the largest of the world religions; it is estimated that about a third of the people in the world profess a Christian faith.
Christianity emerged from Judaism in the first century of the common era. Christians brought from Judaism its scriptures; fundamental doctrines such as monotheism; the belief in a moshiach (Hebrew term for messiah; this term is more commonly known as Christ (christos in Greek) and means 'the annointed one'); form of worship, including a priesthood, concepts of sacred space and sacred time, the idea that worship here on Earth is patterned after worship in Heaven, and the use of the Psalms in community prayer. The book of Acts says that Christ's followers were first called Christians by non-believers in the city of Antioch, where they had fled and settled after early persecutions in Palestine, probably just a few years after Jesus' resurrection and ascension.
The Jewish conception of the messiah is significantly different than that held by Christians. According to Jews, the Hebrew Bible contains a small number of prophecies concerning a future descendent of King David, who will be annointed (Hebrew: moshiach) as the Jewish people's new leader. In the Jewish view, this fully human and mortal leader will rebuild the land of Israel and restore the Davidic Kingdom. This subject is covered in the section on Jewish eschatology. Christian understandings of the term "messiah" are based on Jesus' statements about himself in the New Testament, namely: (a) that he was the fulfillment of many Old Testament prophecies, (b)that he came to establish the Kingdom of God (or Kingdom of Heaven), which was not to be an earthly kingdom, (c) that when asked straight in the face whether he was the expected messiah, he pointed at the miracles he performed and (d) that by demonstratively washing the disciples's feet he pictured himself to be a 'servant-king'. (See Judeo-Christian tradition and Comparing and Contrasting Judaism and Christianity)
Doctrine
The most crucial points in Christian teaching are Jesus' incarnation, crucifixion and death, and his miraculous resurrection. These events are believed by Christians to be the basis of God's work to reconcile humanity with himself. Most Christian groups believe Jesus to be God (ie: incarnated as both fully human and divine), although there has been never-ending dispute over in what way he is God (see Christology). A few Christian groups (Arians, Jehovah's Witnesses, Unification Church) have denied that he is God, saying instead that he is a created being second only to God. Gnostic Christians, which are no longer extant as a group, argued that he was fully God, but only seemed to be fully human. In a few modern day liberal Protestant Christian sects, Jesus is not believed to be God, but rather is viewed as someone who had new insights and something to teach; this position has only been accepted by a minority of Christians. The stated doctrines of Catholic, Orthodox, and almost all Protestant churches hold that Jesus was both fully God and fully man, and hold this to be an essential element of the faith.
Christians believe the Bible is the word of God. Even though Christians disagree about how accurate the Bible is and how it should be interpreted, it is still the most widely regarded source of information about Jesus and God. The New Testament maintains that Jesus is the messiah which the Jews have long awaited; thus Christianity is considered by Christians to be the continuation or fulfillment of the Jewish faith. Christians and Jews both consider the Old Testament (what Jews call the Tanakh) to be the word of God. Some Christians include additional books, but most agree on which books comprise the Old Testament (see biblical canon). The New Testament is the second part of the Christian Bible, containing accounts of the life of Jesus, the earliest church, and a number of epistles (letters) written by some of the Apostles to various audiences.
Denominations
The greatest division in Christianity is between the eastern and western branches. The Western branch developed in the Western Roman Empire, while the Eastern branch developed in the Eastern Roman Empire. The Western branch is divided principally into Catholicism and Protestantism, while the two main divisions of the Eastern branch are Eastern Orthodoxy and Oriental Orthodoxy. See the Great Schism for the history and circumstances of this division.
Western branches
Catholicism and Protestantism are the two major divisions of Christianity in the Western world (Western Europe and the Americas). For example, the Baptist, Methodist, and Lutheran churches are generally considered to be Protestant faiths, although strictly speaking, of these three the Lutheran denomination is the only one of these founded as a "protest" against Catholicism. The Anglican (Church of England) is generally classified as Protestant, but it is properly understood as its own tradition—a via media ("middle way") between the Protestant and Catholic traditions.
One central tenet of Catholicism is its literal adherence to apostolic succession. "Apostle" means "one who is sent out." Jesus commissioned the first twelve apostles (see Biblical Figures for the list of the Twelve), and they, in turn laid hands on subsequent church leaders to ordain (commission) them for ministry. In this manner, Catholics can trace their ordained ministers all the way back to the original Twelve. Roman Catholics are distinct in their belief that the Pope has authority which can be traced directly to the apostle Peter. Other Catholic groupings include the Old Catholic Church which rejected the definition of papal infallibility at the First Vatican Council, and Anglo-Catholics, Anglicans who believe that Anglicanism is a continuation of historical Catholicism and who incorporate many Catholic beliefs and practices.
Protestant faiths trace their roots to the work of Martin Luther and John Calvin, who believed that the Catholic church had deviated too far from the practices and beliefs of the original churches described in the New Testament. They attempted to reform the Catholic Church but failed. The protestant reformation resulted instead. Protestantism as a whole has never been led by a pope or other institution having such an over-all authority. Each protestant movement has developed freely, and many have split over theological issues. That is how over the centuries it has developed into a great number of independent denominations. A number of movements that grew out of spiritual revivals, like Methodism and Pentecostalism, also consider themselves Protestant. The measure of mutual acceptance between the denomonations and movements varies, but is growing. Protestant theology for each denomination is usually guarded by church councils.
Eastern branches
In the Eastern world (Eastern Europe, Asia) the primary representative of Christianity is Eastern Orthodoxy. The Eastern Orthodox Church also believes it is the continuation of the original Christian church established by Christ. Originally there were five main centers of Christianity in the ancient world: Rome, Constantinople, Alexandria, Antioch, and Jerusalem. According to the Eastern Churches' understanding of Papal primacy, the bishop of Rome was first in honor among the bishops, but possessed no direct authority over dioceses other than his own. In the Great Schism, conventionally dated to 1054, the Eastern Churches severed communion with Rome over a number of issues centered around the differing understanding of Papal primacy. The four other Churches remained in communion with each other and still exist today along with less prestigious, but often more populous, self-governing or "autocephalous" Churches organized more or less along national lines. The largest of these, and the largest Orthodox Church overall, is the Church of Russia. Many of these groups are represented as independent ecclesiastical bodies in America. There exist significant theological differences between the Orthodox Church and Western Christianity.
The Eastern Orthodox Churches accepted the Chalcedonian dogma on the nature of Christ, which was also accepted by the Western branch of the church; while the Oriental Orthodox rejected it. The Oriental Orthodox comprise chiefly the Monophysites (e.g. the Coptic church or the Syrian Jacobites) and the Nestorians (e.g. the Assyrian Church).
Other branches
Several other faiths, which also believe in Jesus Christ, claim not to be descended from any of these groups directly. The Church of Jesus Christ of Latter-day Saints, for instance, is often grouped with Protestant religions, but disagrees with this classification.
Many people belonging to the above-listed groups have strong feelings about the legitimacy of the other Christian faiths, sometimes even claiming that the other faiths do not actually count as Christian. These claims usually rely on more specialized definitions of "Christianity" than an outsider might assume, entailing points of doctrine which are critical to the objecting faith's view, such as belief in the doctrine of the Holy Trinity (an agreement on the nature of God the Father, Jesus Christ, and the Holy Spirit reached in A.D. 325) or the doctrine called "Biblical inerrancy" (a specific set of beliefs regarding the nature of the Bible). Catholics and Protestants in particular may consider these beliefs critical to "real" Christianity, and may thus exclude churches such as The Church of Jesus Christ of Latter-day Saints, the Jehovah's Witnesses and the Unification Church who differ on these views (but feature Jesus Christ as a central part of their belief system, and consider themselves Christian). Others, such as Unitarian-Universalists, consider themselves as borderline Christians, since Jesus Christ is not pivotal to their belief system.
In addition, Christianity has inspired other religions whose adherents do not consider themselves Christians but do consider Jesus to be a prophet. Islam is the most sucessful of these.
Relations with other faiths
Historically, a number of Christians have practiced Anti-semitism. The type, severity and extent of it has varied much over time. Many times in the last 2000 years anti-Semitism has been widely accepted and promulgated by Christian leaders and laypersons; other times it has been condemned. In recent years there has been much to note in the way of reconciliation between Christians groups and the Jewish people. This topic is discussed in more detail in the entry on Christian_anti-Semitism.
(Relations with Hinduism, Buddhism, Islam, atheism / humanism & freethought, and other faiths should be written about here.)
Summary
The following ASCII art diagram shows the historical development of traditional Christian groups:
Protestantism
Reformation--> /
(16th c.) /
-------( Roman Catholicism (Western Rites)
/ ( Roman Catholicism (Eastern Rites)
Early Western Church /
Christianity .....................<--- Great Schism (11th c.)
Eastern Church
------- Eastern Orthodoxy
Chalcedonian -->
Controversies --Nestorians ) Oriental
(5th c.) ---Monophysites ) Orthodox
Christianity today
Not all people who identify as Christians accept all, or even most, of the theological positions that their particular church mandates. Like the Jewish people, Christians in the West were greatly affected by The Enlightenment in the late 17th and early 18th centuries. Perhaps the most significant change for them was total or effective separation of Church and State, thus ending the state-sponsored Christianity that existed in so many European countries. Now one could be a free member of society and disagree with one's church on various issues, and one could even be free to leave the church altogether. Millions did take these paths, becoming freethinkers and developing entirely new belief systems such as humanism, atheism, agnosticism, and deism; others created liberal wings of Protestant Christian theology, and the long-suppressed Unitarian trend in Christitianity became an acceptable choice for many. The Enlightenment had a much less profound impact on the Eastern Churches of Eastern and Oriental Orthodoxy.
This gain in personal freedom came with a social price: the dissolution of the Christian community as an entity with civic legal authority. In the United States and Europe, many secularized Christians have long since stopped participating in traditional religious duties, attending churches only on a few particular days per year or not at all. Many of them recall having highly religious grandparents, but grew up in homes where Christian theology was no longer a priority. They have developed ambivalent feelings towards their religious duties. On the one hand they cling to their traditions for identity reasons; on the other hand, the influence of the secular Western mentality, the demands of daily life, and peer pressure tear them away from traditional Christianity. Marriage between Christians of different denominations, or between a Christian and a non-Christian, was once taboo, but has become commonplace.
There have been many responses to this phenomenon within the Christian community, including the development of literally thousands of Christian Protestant denominations, traditionalist splinter groups of the Catholic Church that do not recognize the legitimacy of many reforms the Catholic Church has undertaken, and the growth of hundreds of fundamentalist groups that interpret the entire Bible in a literal fashion.
For the full history of Christianity, see the article so titled.
The Persecution of Christians, both in the past and today, is the subject of a separate entry.
References
See also: history of Christianity, Christian eschatology, eschatology, the stories of Christianity, missions, missionary, History of Christian Missions, predestination, Great Schism
Main branches of Christianity today: Catholicism -- Protestantism -- Eastern Orthodoxy -- Oriental Orthodoxy -- (see below for exceptions)
Eastern Orthodox Churches: Russian Orthodox Church -- Greek Orthodox Church -- Serbian Orthdodox Church -- Romanian Orthodox Church -- Antiochian Orthodox Church -- Orthodox Church in America
Oriental Orthodox Churches: Coptic Church -- Armenian Orthodox Church
Protestant families of denominations: Presbyterian Church -- Anglican (Episcopal) Church -- Lutheran Church -- Baptist Church -- Methodist Church -- Pentecostal -- Brethren -- Seventh-day Adventist Church
Protestant theologies: Calvinism -- Arminianism
Non-Protestant, non-Catholic Christianity: The Church of Jesus Christ of Latter-day Saints -- Jehovah's Witnesses -- Unification Church -- Religious Society of Friends (Quakers) -- Church of Christ, Scientist (Christian Scientists) -- Sons Aumen Israel -- Unity Church -- Christadelphians
Unitarian Universalism is not a creedal religion, and espouses no official spiritual beliefs. Unitarian Universalists see themselves as a liberal religious community where people of differing beliefs share with each other and learn from each other. Before the 20th century, the church was much more Christian, but it has become increasingly diverse.
Ancient (largely extinct) Christian groups: Gnosticism -- Montanism -- Arianism -- Docetism (often excluded from classification as Christians by modern theologians)
Links to integrate with the above (taken from religion): Lutheranism -- Presbyterianism -- Evangelicalism -- Church of Christ -- Charismatics and many more.
A summary of Christian views of homosexuality is available..
Relevant books: The Rise of Christianity (book by Rodney Stark)
Specific commentary about this article can be found on /Talk. General commentary about Christianity moved to ChristianityTalk. Let's avoid stating idiosyncratic and biased views on the main pages.