The doctrine of the Trinity, considered the core of Christian theology by Trinitarians, is the result of continuous exploration by the church of the biblical data, thrashed out in debate and treatises, eventually formulated at the First Council of Nicaea in AD 325 in a way they believe is consistent with the biblical witness, and further refined in later councils and writings.Oxford Dictionary of the Bible, Trinity Article The most widely recognized Biblical foundations for the doctrine's formulation are in the Gospel of John, which possess ideas reflected in Platonism and Greek philosophy.Hurtado, Larry. Lord Jesus Christ: Devotion to Jesus in Earliest Christianity. Eerdmans, 2003, 366–367

Nontrinitarianism is any of several Christian beliefs that reject the Trinitarian doctrine that God is three distinct persons in one being. Modern nontrinitarian groups views differ widely on the nature of God, Jesus, and the Holy Spirit.

Historical theology is the academic study of the development of Christian theology.

Background Second Temple Judaism

Christianity originated as a sect within Second Temple Judaism (516BC – AD70). Judaism's sacred scripture is the Hebrew Bible, known to Christians as the Old Testament. The Hebrew Bible is divided into three parts: the Torah ("Law"), the Nevi'im ("Prophets") and the Ketuvim ("Writings"). The central belief of Judaism is monotheism: there is one God named Yahweh (Deuteronomy 6:4, Isaiah 44:6).

Israel

According to the Bible, God promised Abraham, the Jewish patriarch, that his descendants would become a great nation and a blessing to all the earth (Genesis 12:3). Therefore, the Jews were a chosen people (Deuteronomy 7:6–8) bound by a covenant with God. God's election placed certain obligations on the Jewish people, and these () were found in the Torah. They were to love Yahweh with all their hearts and worship no other gods (Deuteronomy 6:4–5). Likewise, they were commanded to love one another (Leviticus 19:18). In addition, they were to observe the Sabbath, follow kosher food laws, and circumcise their sons. By obeying the commandments, Israel would be a holy nation, reflecting God's holiness to the world and manifesting the kingdom (rule) of God. Possession of the land of Israel was contingent on fidelity to the Torah, and the House of David was considered the rightful rulers over the land (2 Samuel 7:11–16).

The Temple in Jerusalem was the dwelling place and throne of God (1 Kings 8:12–13). It was there that a hereditary priesthood offered sacrifices of incense, food, and various kinds of animals to God. The sin offering (Leviticus 4–5) and the annual Day of Atonement ritual (Leviticus 16) served to both purify the temple and atone for individual and national sins. "In both cases the sacrificial blood made possible the continuation or restoration of the relation between God and his people ruptured by sin and/or prevented by human impurity."

Conquest and Hellenization

The First Temple was destroyed during the siege of Jerusalem in 587BC, and the Jewish people were carried away into the Babylonian exile (586–538BC). With the loss of the land and the temple, studying and following Torah became the central concern of Judaism. Some Jews, particularly the Pharisees, came to believe that Torah devotion could replace temple worship, and this was facilitated by the establishment of synagogues. For those unable to offer sacrifices at the temple, spiritual sacrifices (almsgiving, prayer, fasting, or Torah study) could be substituted. After the return from exile, the Second Temple became the center of Jewish national and religious life; nevertheless, the Torah continued to serve as "a portable Land, a movable Temple".

After the conquests of Alexander the Great (356–323BC), the Jewish homeland experienced Hellenization (the spread of Greek culture). Hellenistic Judaism was a movement that sought to combine Judaism with the best elements of Greek culture. The Greek-speaking Jews of Alexandria produced a Greek translation of the Bible called the Septuagint, which was used by the first Christians.

Antiochus IV Epiphanes (), king of the Seleucid Empire, adopted anti-Jewish policies in an attempt to Hellenize the Jews. He prohibited Jewish religious practices in 168 BC, introduced the worship of Zeus in the Temple in 167BC (the "abomination of desolation" of Daniel 9:27 and Daniel 11:31), and set up pagan altars everywhere. Jews who refused to follow the king's religious policies faced death. Jewish martyrdom contributed to the belief in resurrection of the dead (2 Maccabees 7:9).

The Maccabean Revolt in 167BC restored an independent Jewish kingdom under the Hasmonean dynasty, which ruled as kings and high priests. However, some Jews believed the Hasmoneans lacked legitimacy since they were not descended from the royal line of David nor the priestly line of Zadok. The Roman Empire conquered the Hasmonean kingdom in 63BC and appointed client kings. Under the Romans, the formerly lifelong office of high priest was filled by temporary appointments and "became increasingly the plaything for political and financial interests."

Apocalypticism

Jewish writings offered different explanations for Israel's domination by foreigners. According to the prophets Isaiah, Jeremiah, Amos, and Hosea, Israel suffered as punishment for its sins. If the people repented, God would restore the kingdom.

An alternative worldview known as apocalypticism developed around the time of the Maccabean Revolt. The term is derived from the Greek word ( or ), and Jewish apocalypticists believed God had revealed the future to them. Major texts of apocalyptic literature were Joel, 1 Enoch, Daniel, the Isaiah apocalypse (chapters 24–27), Jubilees, Ascension of Moses, Sibylline Oracles, 4 Ezra, 2 Enoch, 2 Baruch, Apocalypse of Abraham, some of the Dead Sea Scrolls, and 3 Baruch.

Apocalypticism is a dualistic worldview. In the present age, the world is corrupted by the cosmic forces of evil (Satan and his demons). But in the future, God and his angels will destroy the forces of evil. God will redeem the whole world, not just the Jewish people. In the age to come, "there would be no suffering or pain; there would be no more hatred, despair, war, disease, or death. God would be the ruler of all, in a kingdom that would never end." At this time, there will be a last judgment and a universal resurrection so that all people can be rewarded or punished.

Apocalypticists expected God to accomplish these things by sending a savior figure or messiah. Messiah (Hebrew ) means "anointed" and was used in the Old Testament to designate Jewish kings and in some cases priests and prophets whose status was symbolized by being anointed with holy anointing oil. The term is most associated with King David, to whom God promised an eternal kingdom (2 Samuel 7:11–17). After the fall of David's dynasty, this promise was reaffirmed by the prophets Isaiah, Jeremiah, and Ezekiel, who foresaw a future Davidic king who would establish and reign over an idealized kingdom.

In the Second Temple period, there was no consensus on who the messiah would be or what he would do. Some believed the messiah would be a political figure, a son of David, who would restore an independent Kingdom of Israel. A detailed description of such a Davidic messiah is found in the 17th psalm of the Psalms of Solomon. Others believed in a priestly messiah. Psalm 110 refers to a figure belonging to the priesthood of Melchizedek, the priest-king of Genesis 14:17–24. The Testament of Levi refers to God raising up a "new priest". Others anticipated the coming of a celestial figure called the Son of Man who would lead armies of angels and inaugurate the reign of God on earth (Daniel 7:13–14).

Jewish sects After the Maccabean Revolt, several Jewish sects emerged that differed on their interpretations of the Torah, their reaction to foreign domination, and their acceptance of a non-Zadokite high priest. The New Testament explicitly mentions three Jewish sects, and the Jewish historian Josephus mentions four.

The Pharisees rejected Greek religion and culture. They were committed to obeying the Torah and developed rules to clarify its ambiguities. For example, the Ten Commandments required Jews to keep the Sabbath holy, but the Torah does not explain how this is to be done. The rules developed by the Pharisees became known as the oral Torah. Like priests serving in the Temple, Pharisees attempted to maintain a state of Temple holiness throughout their daily lives by adhering to Jewish purity laws. This required them to separate from ordinary people and could explain the origin of the term Pharisee, which means "separated ones". As a lay movement, the Pharisees had no official religious role within Judaism, but they were respected by ordinary Jews. Pharisaic ideas, such as belief in angels, demons, and resurrection became widely accepted.

The Sadducees were drawn from the families of the chief priests and the Jewish aristocracy. Their religion centered on the Temple and its sacrifices, and they cooperated with the Romans so that those sacrifices could continue. They were less concerned with purity regulations in daily life. The Sadducees only accepted the written Torah as authoritative, and they denied the existence of angels or a future resurrection.

The Essenes formed in protest of the Hasmonean usurpation of the high priesthood and refused to worship at the Jerusalem Temple. Separating from other Jews, they were an ascetic movement focused on strict adherence to the Torah and purity regulations. Like the Pharisees, Essenes believed in the existence of angels and in a future resurrection. The Essenes were an apocalyptic sect that believed God would give Israel two messiahs—a Davidic king and an anointed priest—to lead the Sons of Light in a war against the Sons of Darkness . The Sons of Light will be victorious, and a new Temple will be built to replace the corrupt one.

Josephus writes about an unnamed "fourth philosophy" that could include several different groups, particularly the Zealots and the Sicarii. These groups endorsed armed resistance against foreign rulers and their Jewish collaborators.

Greek philosophy

Church historian Diarmaid MacCulloch has observed that "Christianity in its first five centuries was in many respects a dialogue between Judaism and Graeco-Roman philosophy". There are similarities between early Christianity and ancient philosophy. Biblical scholar Udo Schnelle writes, "philosophy is a salutary means for living well and dying well... The followers of Jesus of Nazareth practiced a comparable lifestyle, treated comparable subjects, and produced comparable literature."

The related philosophies of Cynicism and Stoicism emphasized ethics and individual freedom. Cynics "commended and practiced a lifestyle oriented to nature and to reason, which eradicates false passions from the soul (lust, craving, anger) and leads to a simple life without needs." Some Cynics adopted a wandering, ascetic way of life that may have influenced Christian asceticism.