It took some 350 years to get from the Apostolic Age to the doctrinal formulations of the Nicene Creed. The question thus arises whether a process of development was taking place. If so, what kinds of development were they? What was their significance, both for the substantive issues affected and for the way in which the formative period is viewed by subsequent generations of Christians? And is a principle of development allowed or established that may then be applied to other issues and at other times?

As the 2nd century turned into the 3rd, both Irenaeus, in Against Heresies, and Tertullian, in On the Prescription of Heretics, in reference to the variability, innovations, and secretiveness of the teaching of the so-called gnostics, pointed to the constant and public teaching given throughout the church, notably in the apostolic sees, and most particularly in Rome, where the church was founded by Peter and Paul. In setting out the “rule of faith,” Irenaeus combines a recital of the mighty acts of God in creation and history with the threefold structure of the divine Name in which baptism is administered (Matthew 28:19, and the baptismal profession found in the Apostolic Tradition).

The rule of faith outlined by Irenaeus and Tertullian remains the formal pattern of the Nicene Creed. However, the evolution of doctrine between their time and the 4th-century councils of Nicaea and Constantinople is suggested by the insertions that the two councils made in the older texts concerning the essential being of the Son (“God from God, Light from Light, True God from True God, begotten not made, of one substance with the Father”) and of the Holy Spirit (“the Lord, the Giver of Life, who proceeds from the Father, who with the Father and the Son together is worshiped and glorified”). These steps were taken in order to safeguard the established soteriological understandings and liturgical practices against rather blatant distortions of the apostolic message, and as a result of the exploration of previously unposed or unsettled questions and the intellectual and spiritual energy of successive generations in applying the inherited faith within their cultural circumstances.

This was the kind of process that John Henry Newman called “the development of an idea.” As noted in his Essay on the Development of Christian Doctrine, a “great idea” takes a “longer time and deeper thought for [its] full elucidation,” but this process of “germination and maturation” will be a “development” only if “the assemblage of aspects, which constitute its ultimate shape, really belongs to the idea from which they start.” “Young birds do not grow into fishes,” said Newman in that work.

Newman also thought that such a development would continue, and he left the Anglican church for the Roman Catholic Church in 1845 because he judged that the latter best embodied such a development. It would in fact be developmental grounds that provided a theological justification for the doctrines of Mary’s Immaculate Conception (defined in 1854) and heavenly assumption (defined in 1950). The declaration of these teachings was held to make explicit things that were implicit in the apostolic witness but had required centuries of devotional practice and speculative reasoning to be brought out. Newman also considered that an infallible teaching office lay in the origins and logic of a developmental Christianity—indeed with a Roman focus, although he questioned the “opportunity” of its dogmatization in 1869–70, which in substance attributed that function to the pope without a general council.

The Eastern Orthodox churches also accept a development of doctrine beyond Nicaea I and Constantinople I, embracing not only the Council of Ephesus in 431 (as do the Oriental Orthodox churches) but also the Councils of Chalcedon, Constantinople II and III, and Nicaea II. The later councils are viewed as having clarified and explicated, but not altered, the teachings of the earlier councils. Thus, Nicaea II, for instance, in deciding for the veneration of icons, was being true to the dogmas of the one person and two natures of Christ. The Eastern churches also hold to the infallibility of the church, thanks to its divine foundation and guidance by the Son and the Spirit and the pastoral oversight of its bishops in faithful succession. They do not, however, judge that the conditions have been met for the meeting of an ecumenical council after Nicaea II and the reception of its teaching by the whole body of the faithful. This has not stopped certain “doctrinal developments” from being widely regarded as legitimate and commendable. An example is the reception of the teaching of Gregory Palamas (14th century), who identified the “uncreated light” manifested at the Transfiguration of Christ on Mount Tabor with the “divine energies” by which Christian believers are savingly “deified” (an inner transformation mystically uniting God and the individual).

The Protestant reformers in the 16th century attempted to undo what they regarded as false developments (“corruptions,” in Newman’s terminology) in the Western church. They wished to go back—not so much historically as theologically—to Scripture, especially in matters of applied soteriology (though in matters of Christology and the Trinity they remained under the guidance of the councils of the 4th and 5th centuries). Modern progressive Protestants sometimes try to reclaim the notion of development to justify certain recent shifts that others would regard as deviations or degeneration.