First Council of Nicaea in the context of "Mainstream Christian"

In the history of Christianity, the first seven ecumenical councils are as follows: the First Council of Nicaea in 325, the First Council of Constantinople in 381, the Council of Ephesus in 431, the Council of Chalcedon in 451, the Second Council of Constantinople in 553, the Third Council of Constantinople from 680 to 681 and finally, the Second Council of Nicaea in 787.

These seven events represented an attempt by Church leaders to reach an orthodox consensus, restore peace and develop a unified Christendom. Among Eastern Christians the Eastern Orthodox, Oriental Orthodox, and Church of the East (Assyrian) churches and among Western Christians the Roman Catholic, Anglican, Utrecht and Polish National Old Catholic, and some Scandinavian Lutheran churches all trace the legitimacy of their clergy by apostolic succession back to this period and beyond, to the earlier period referred to as the Early Church.

Patriarchate (/ˈpeɪtriɑːrkɪt, -keɪt/, UK also /ˈpætri-/; Ancient Greek: πατριαρχεῖον, patriarcheîon) is an ecclesiological term in Christianity, referring to the office and jurisdiction of a patriarch.

According to Christian tradition, three patriarchates—Rome, Antioch, and Alexandria—were established by the apostles as apostolic sees in the 1st century. These were officially recognized by the First Council of Nicaea. The Patriarchate of Constantinople was added in the 4th century, and the Patriarchate of Jerusalem followed in the 5th century. These five sees were later recognized collectively as the pentarchy by the Council of Chalcedon in 451.

Augustine of Hippo (/ɔːˈɡʌstɪn/ aw-GUST-in, US also /ˈɔːɡəstiːn/ AW-gə-steen; Latin: Aurelius Augustinus Hipponensis; 13 November 354 – 28 August 430) was a theologian and philosopher, the bishop of Hippo Regius from Thagaste in Numidia Cirtensis, (present-day Souk Ahras, Algeria). His writings deeply influenced the development of Western philosophy and Western Christianity, and he is viewed as one of the most important Church Fathers of the Latin Church in the Patristic Period. His many important works include The City of God, On Christian Doctrine, and Confessions.

According to his contemporary, Jerome of Stridon, Augustine "established anew the ancient Faith". In his youth he was drawn to the Manichaean faith, and later to the Hellenistic philosophy of Neoplatonism. After his conversion to Christianity and baptism in 386, Augustine developed his own approach to philosophy and theology, accommodating a variety of methods and perspectives. Believing the grace of Christ was indispensable to human freedom, he helped formulate the doctrine of original sin and made significant contributions to the development of just war theory. When the Western Roman Empire began to disintegrate, Augustine imagined the Church as a spiritual City of God, distinct from the material Earthly City. The segment of the Church that adhered to the concept of the Trinity as defined by the Council of Nicaea and the Council of Constantinople closely identified with Augustine's On the Trinity.

Early Christianity, otherwise called the Early Church, describes the historical era of the Christian religion up to the First Council of Nicaea in 325. Christianity spread from the Levant, across the Roman Empire, and beyond. Originally, this progression was closely connected to already established Jewish centers in the Holy Land and the Jewish diaspora throughout the Eastern Mediterranean. The first followers of Christianity were Jews who had converted to the faith, i.e. Jewish Christians, as well as Phoenicians, i.e. Lebanese Christians. Early Christianity contains the Apostolic Age and is followed by, and substantially overlaps with, the Patristic era.

The Apostolic sees claim to have been founded by one or more of the apostles of Jesus, who are said to have dispersed from Jerusalem sometime after the crucifixion of Jesus, c. 26–33, perhaps following the Great Commission. Early Christians gathered in small private houses, known as house churches, but a city's whole Christian community would also be called a "church"—the Greek noun ἐκκλησία (ekklesia) literally means "assembly", "gathering", or "congregation" but is translated as "church" in most English translations of the New Testament.

Nicene Christianity includes those Christian denominations that adhere to the teaching of the Nicene Creed, which was formulated at the First Council of Nicaea in AD 325 and amended at the First Council of Constantinople in AD 381. It encompasses the vast majority of today's Christian churches.

Heresy is any belief or theory that is strongly at variance with established beliefs or customs, particularly the accepted beliefs or religious law of a religious organization. A heretic is a proponent of heresy.

Heresy in Judaism, Christianity, and Islam has at times been met with censure ranging from excommunication to the death penalty.

The Nicene Creed, also called the Creed of Constantinople, is the defining statement of belief of Nicene Christianity and in those Christian denominations that adhere to it.

The original Nicene Creed was first adopted at the First Council of Nicaea in 325. According to the traditional view, forwarded by the Council of Chalcedon of 451, the Creed was amended in 381 by the First Council of Constantinople as "consonant to the holy and great Synod of Nice." However, many scholars comment on these ancient Councils, saying "there is a failure of evidence" for this position since no one between the years of 381–451 thought of it in this light. Further, a creed "almost identical in form" was used as early as 374 by St. Epiphanius of Salamis. Nonetheless, the amended form is presently referred to as the Nicene Creed or the Niceno-Constantinopolitan Creed.

The Babylonian calendar was a lunisolar calendar used in Mesopotamia from around the 2nd millennium BC until the Seleucid Era (294 BC), and it was specifically used in Babylon from the Old Babylonian Period (1780s BC) until the Seleucid Era.

In the Seleucid Era it was reformed as "Greek time", Anno Graecorum was introduced and used in the Middle East and Egypt until the middle of the first millennium when the First Council of Nicaea AD 325 defined the Church year based on the Roman early Julian calendar. As Anno Graecorum formed the basis for time references in the Bible and spread westward, it rather increased the Babylonian calendars importance. The Babylonian calendar is also partly reflected in calendars in South and East Asia and the Islamic calendar as well as Iranian calendars. The Julian calendar inherited the definitions of the 12 month system, week, hour etc. from the Babylonian calendar and the current Jewish calendar can be seen as a slightly modified Babylonian calendar that still exists today and is practised, but with Anno Mundi Livryat haOlam year calculation since the creation of the world. Today's global time system UTC (Gregorian calendar) therefore has its main structure inherited from the Babylonian calendar.