The concept of orthodoxy denotes a central set of doctrines, often specified by a recognised authoritative body or set of individuals, to which any person must subscribe in order to be accepted by others as a fellow member of a religious community. Despite some possible precedents among ancient philosophers, the concept of orthodoxy developed in a distinct manner within Christianity in tandem with the notion of heresy, especially from the 2nd century onward. This involved defining an identity around certain core beliefs, alongside particular practices, apostolic traditions, and canonical texts, thereby gradually restricting the boundaries of theological speculation and acceptable difference of opinion. This was partly in response to disagreements with people who regarded themselves, or were most regarded by others, as forming part of the religious community, but also partly in response to criticisms from non-Christians. These arguments were, therefore, focused on the establishment of identity for a group through the establishment of boundaries, particularly in a context of a diversity of scattered Christian communities in which there was no recognised central authority to which adherents could appeal. Some of these earliest disputes were centred on the status of the Creator God and other cosmological issues, but also included Christological disagreements concerning Jesus Christ himself, which would go on to be the most prominent sources of controversy in the attempts to define orthodoxy during late antiquity. In the 4th century, the central issues concerned Trinitarian doctrine and the question of how to define the relationship between the Father, the Son, and the Holy Spirit. From the early 5th century onward, the problem of the humanity and divinity of Christ came to the fore. Bishops and other authors sought to define orthodoxy and persuade other Christians through a range of methods including preaching, the writing of theological treatises, often citing relevant passages of Scripture, the exchanging of letters, the composition of lists of heresies (known as heresiologies), and the calling of local ecclesiastical councils.
From the reign of Constantine I onward, the situation changed substantially, since imperial support for Christianity and the institution of the Church created new opportunities, rewards, and dangers for those involved in arguments concerning orthodoxy. Emperors were often directly involved in seeking solutions to these disputes, including through the calling of larger councils attended by bishops from across the empire. Those held at Nicaea in 325, Constantinople in 381, Ephesus in 431, Chalcedon in 451, and Constantinople in 553 have come to have the status of ecumenical councils for many Christians, meaning that they included representatives from both the “eastern” and “western” parts of the Roman empire and that their decisions applied to the whole oikoumene and had universal validity. Nonetheless, other large gatherings also took place, including the dual council of Seleucia-Ariminum in 359, which are not celebrated because they proposed theological definitions that did not go on to be accepted as orthodox. The decisions reached at these events were often supported by imperial actions, such as the exiling of those who had been deposed and excommunicated. From the later 4th century onward, statements of orthodoxy increasingly came to be enshrined in laws, as emperors sought to enforce their preferred definitions of faith on their subjects. Dialogue literature also become more popular as a means of articulating theological positions and arguments, while at the same time disputants made increasing use of collections of statements by authoritative churchmen known as florilegia. By the end of antiquity, a clear concept of orthodoxy had emerged, but there continued to be significant disagreements about how it should be defined, especially with the fracturing of Roman imperial territory.
