Diversity in early Christian theology in the context of "State church of the Roman Empire"

Christianity in the ante-Nicene period was the period in Christian history following the Apostolic Age (1st century AD) up to the First Council of Nicaea (325 AD). Although the use of the term Christian (Koine Greek: Χριστιανός) is attested in the Acts of the Apostles (80–90 AD), the earliest recorded use of the term Christianity (Koine Greek: Χριστιανισμός) is attested by the ante-Nicene Father and theologian Ignatius of Antioch (c. 107 AD).

While the Jewish–Christian community was centered in Jerusalem in the 1st century AD, Gentile Christianity spread widely in the 2nd century AD. One stream of Gentile Christianity (so-called "proto-Orthodox Christianity") that emerged in this period in the persons and theological positions of the Apostolic Fathers would eventually become the international Great Church. Proto-Orthodox Christianity placed importance on the sacrifice of Jesus on the cross as saving humanity, and described Jesus as the incarnated Son of God come to Earth. The 2nd and 3rd centuries AD saw a sharp separation between Jewish Christianity and Gentile Christianity, with the latter being derived from the teachings of the Apostle Paul. There was an explicit rejection of Second Temple Judaism and Jewish culture by the end of the 2nd century, with a growing body of anti-Jewish Christian literature. Many doctrinal variations in this era defy neat categorizations, as various forms of Christianity interacted in a complex fashion.

Adoptionism, closely associated with dynamic monarchianism, is an early Christian nontrinitarian theological doctrine, subsequently revived in various forms. Adoptionism is a theology about relationship, which does not affirm the virgin birth and holds that Jesus was adopted as the Son of God at his baptism, his resurrection, or his ascension. Dynamic Monarchianism does not hold Jesus’ sonship was through adoption. Dynamic monarchianism is a theology about divinity, and holds there is one God, the Father, and Jesus was a man, born of virgin birth, empowered by God’s own divinity through the Holy Spirit, and raised to unity with God, but not a pre-existent distinct divine person. How common adoptionist views were among early Christians is debated, but it appears to have been most popular in the first, second, and third centuries. Some scholars see adoptionism as the belief of the earliest followers of Jesus, based on the epistles of Paul and other early literature. However, adoptionist views sharply declined in prominence in the fourth and fifth centuries, as Church leaders condemned it as a heresy.

Gaston (2023) defines a distinction between adoptionism and dynamic monarchianism as different christologies, while being commonly conflated. Adoptionism refers to a theological position about the relationship between Jesus and the Father (I.e. that he was adopted by God), not the deity of Jesus, while Dynamic Monarchianism is a Christological position about the deity of Jesus. In Dynamic Monarchiansim, “Dynamic” refers to being empowered by the Holy Spirit, while “Monarchianism” emphasizes a single deity (i.e. the Father). Gaston writes that Dynamic Monarchianism describes a Christology that was among the earliest Christologies, held by independent Theologians and maintained within mainstream Christianity until the fourth Century. Dynamic Monarchianism held that Jesus was a miraculously conceived man who, after his resurrection, ascended to heaven and to divine authority, as opposed to being an eternal divine Person who became human. Monarchianism is in contrast to Logos christologies of the second and third century, which distinguish the Logos as another divine person distinct from the Father. Monarchianism is categorized as Modalistic (where Father and Son are different designations for the same person) or Dynamic (where only the Father is God, and Jesus was empowered by his Spirit.

Marcionism was an early Christian dualistic belief system originating with the teachings of Marcion of Sinope in Rome around 144. Marcion was an early Christian theologian, evangelist, and an important figure in early Christianity. He was the son of a bishop of Sinope in Pontus. About the middle of the 2nd century (140–155) he traveled to Rome, where he joined the Syrian gnostic Cerdo.

Marcion preached that the benevolent God of the Gospel who sent Jesus into the world as the savior was the true Supreme Being, different and opposed to the malevolent deity, the Demiurge or creator deity, identified with Yahweh in the Hebrew Bible. He considered himself a follower of Paul the Apostle, whom he believed to have been the only true apostle of Jesus.

Marcion of Sinope (/ˈmɑːrkiən, -siən/; Ancient Greek: Μαρκίων Σινώπης; c. 85 – c. 160) was a theologian in early Christianity. Marcion preached that God had sent Jesus Christ, who was distinct from the "vengeful" God (Demiurge) who had created the world. He considered himself a follower of Paul the Apostle, whom he believed to have been the only true apostle of Jesus Christ; his doctrine is called Marcionism. Marcion published the earliest record of a canon of New Testament books.

Early Church writers such as Justin Martyr, Irenaeus, and Tertullian denounced Marcion as a heretic or antichrist, and he was excommunicated by the church of Rome around 144. He published his own canon of Christian sacred scriptures, which contained ten Pauline epistles (including the Epistle to the Laodiceans, while excluding the Pastoral epistles) and the Gospel of Marcion which historically is claimed to be an edited version of the Gospel of Luke. Some modern scholars, such as Matthias Klinghardt, have theorized that Marcion's Gospel was the oldest, although this has been contested.

Valentinus (Greek: Οὐαλεντῖνος; c. 100 – c. 180 CE) was the best known and, for a time, most successful early Christian Gnostic theologian. He founded his school in Rome. According to Tertullian, Valentinus was a candidate for bishop but started his own group when another was chosen.

Valentinus produced a variety of writings, but only fragments survive, largely those quoted in rebuttal arguments in the works of his opponents, not enough to reconstruct his system except in broad outline. His doctrine is known only in the developed and modified form given to it by his disciples, the Valentinians. He taught that there were three kinds of people, the spiritual, psychical, and material; and that only those of a spiritual nature received the gnosis (knowledge) that allowed them to return to the divine Pleroma, while those of a psychic nature (ordinary Christians) would attain a lesser or uncertain form of salvation, and that those of a material nature were doomed to perish.