Christianity is a religion centered on the life and teachings of Jesus of Nazareth, known by Christians as Jesus Christ, as recounted in the New Testament. Most Christians believe firmly they are monotheistic. With an estimated 2.1 billion adherents in 2001, it is the world's largest religion
Christianity began in the first century as a Jewish sect, and therefore shares many religious texts and early history with Judaism; specifically, the Hebrew Bible, known in the Christian context as the Old Testament .Christianity is considered an Abrahamic religion, along with Judaism and Islam.
In the Christian scriptures, the term "Christian" is first known to have appeared in Acts 11:26: "And in Antioch Jesus' disciples were first called Christians" ( which means "the anointed one").
Christianity began within the Jewish religion among the followers of Jesus of Nazareth. Under the leadership of the Apostles Peter and Paul, it welcomed Gentiles, and gradually separated from Pharisaic Judaism. Some Jewish Christians rejected this approach and developed into various sects of their own, while others were joined with Gentile Christians in the development of the church; within both groups there existed great diversity of belief. Professor Bentley Layton writes, "the lack of uniformity in ancient Christian scripture in the early period is very striking, and it points to the substantial diversity within the Christian religion." A church hierarchy seems to have developed by the time of the Pastoral Epistles (1 Tim 3, Titus 1) and was certainly formalized by the 4th century .
Christianity spread across the Mediterranean Basin, enduring persecution by the Roman Emperors. As Christianity expanded beyond Palestine, it also came into increased contact with Hellenistic culture; Greek philosophy, especially Platonism, became a significant influence on Christian thought through theologians such as Origen. Elements of mystery religions such as Mithraism may have been incorporated into Christianity, although scholars differ on the extent to which the developing Christian faith adopted identifiably pagan beliefs.
Theological disputes about the correct interpretation of Christian teaching led to internal conflicts; the various churches of Early Christianity shared a common creed, but actual beliefs varied widely. By the third century CE, councils were regularly held in provincial capitals to distinguish between orthodox (which literally translates to "right worship") and heretical (or wrong) views. On May 20th, 325 CE, the newly converted Roman Emperor Constantine I convened the First Council of Nicaea which saw the beginnings of a single doctrine through debate and discussion by 220 Bishops and Constantine . Prior to Nicaea, the Western churches did not simply obey Rome and its interpretations, which the Eastern churches still did not . Constantine continued to authoritively control church policy until the day he died , creating a new sense of unity amongst the various churches. After Arianism, the primary target at Nicaea, was declared heretical (despite Arius's attendence), other sects began to be declared heretical. These include Gnosticism, Simonianism, Marcionism, Ebionitism and Montanism. Such disputes, especially in the field of Christology, intensified after the religion's legalization.
Early in the 4th century, the Emperor Constantine the Great legalized Christianity, giving the church a privileged place in society, and in 391 Theodosius I established Nicene Christianity as the official religion of the Roman Empire. From Constantine onwards, the history of Christianity becomes difficult to untangle from the history of Europe (see also Christendom). The Church took over many of the political and cultural roles of the pagan Roman institutions, especially in Europe. The Emperors, seeking unity through the new religion, frequently took part in Church matters, sometimes in concord with the bishops but also against them. Imperial authorities acted to suppress the old pagan cults and groups deemed heretical by the Church, most notably, Arians. The Catholic Encyclopedia notes that "various penal laws were enacted by the Christian emperors against heretics as being guilty of crime against the State. In both the Theodosian and Justinian codes they were styled infamous persons ... In some particularly aggravated cases sentence of death was pronounced upon heretics, though seldom executed in the time of the Christian emperors of Rome."
Various forms of Christian monasticism developed, with the organization of the first monastic communities being attributed to the hermit St Anthony of Egypt around 300. The monastic life spread to many parts of the Christian empire during the 4th and 5th centuries, as many felt that the Christian moral and spiritual life was compromised by the change from a persecuted minority cult to an established majority religion, and sought to regain the purity of early faith by fleeing society.
The Christian Church of the Roman Empire divided into the Latin-speaking west, centered in Rome, and the Greek-speaking east, centered in Constantinople. (There were also significant communities in Egypt and Syria.) Outside the Empire, Christianity was adopted in Armenia, Caucasian Iberia (now Georgia), Ethiopia, Persia, India, and among the Celtic tribes. During the Migration Period, various Germanic peoples adopted Christianity; at first Arianism was widespread (as among Goths and Vandals), but later Roman Catholicism prevailed, beginning with the Franks. The Slavic peoples of Eastern Europe generally adopted Orthodox Christianity, as in the Baptism of Kievan Rus' (988) in Rus' Ukraine (present-day Russia and Ukraine). Cultural differences and disciplinary disputes finally resulted in the Great Schism (conventionally dated to 1054), which formally divided Christendom into the Catholic west and the Orthodox east.
From the 7th century, Christianity was challenged by Islam, which quickly conquered the Middle East and Northern Africa. Numerous military struggles followed, including the Crusades, the Spanish Reconquista and the eventual conquest of the Byzantine Empire and southeastern Europe by the Turks.
Western Christianity in the Middle Ages was characterized by cooperation and conflict between the secular rulers and the Church under the Pope, and by the development of scholastic theology and philosophy. Later, increasing discontent with corruption and immorality among the clergy resulted in attempts to reform Church and society. The Roman Catholic Church managed to renew itself at the Council of Trent (1545–1563), but only after Martin Luther published his 95 theses in 1517. This was one of the key events of the Protestant Reformation which led to the emergence of Christian denominations. During the following centuries, competition between Catholicism and Protestantism became deeply entangled with political struggles among European states, while many Orthodox Christians found themselves living under Muslim rulers.
Partly from missionary zeal, but also under the impetus of colonial expansion by the European powers, Christianity spread to the Americas, Oceania, East Asia, and sub-Saharan Africa. As the European Enlightenment took hold, Christianity was confronted with the discoveries of science (including the heliocentric model and the theory of evolution), and with the development of biblical criticism (linked to the development of Christian Fundamentalism) and modern political ideologies such as Liberalism, Nationalism and Socialism. In the 19th and 20th centuries, important developments have included the rise of Ecumenism and the Charismatic Movement.