As with other dogmatic developments in the life of the Church, the canonization of scripture was largely done in response to heresy.
For example, prior to Marcion (mid-second century), there was little activity on the part of the Church in establishing a closed ‘canon’ or rule of scripture. It was not until St. Irenaeus that the term ‘New Testament’ was used to describe the apostolic writings. Prior to the latter part of the second century (e.g. Justin Martyr), most references to the scriptures were to what we now call the Old Testament.
In the first few centuries of the Church, writings not considered sacred scripture today were then both widely read and distributed. The Shepherd of Hermas, the Epistle of Barnabas, and the letters of St. Clement of Rome were more popular, for example, than books like 2 Peter, James, or Paul’s letter to the Hebrews. These books are even included in some of the uncial codices of the fourth and fifth centuries (Codex Vaticanus, Codex Alexandrinus, and Codex Sinaiticus). Other peculiarities—such as Jude citing 1 Enoch as prophecy—reveal a situation more complex than some might be willing to admit.
All that being said, when it comes to the closing of a Christian canon of scripture, a number of heresies—and the orthodox response—can be considered, along with some other, important historical events:
Marcion (A.D. 140–155)
Marcion taught that the God of the Old Testament was different from the Father of the New, and that Jesus and this ‘demiurge’ of the Old Testament had nothing in common. As a result, he rejected the teachings of the Old Testament—and its God—as incompatible with both Jesus and the Church. As scripture, Marcion only accepted Luke’s Gospel and ten of St. Paul’s letters (excluding the ‘Pastoral Epistles’). His perspective on divine revelation led to a response from the Church, primarily through St. Irenaeus. Irenaeus, Justin Martyr, and other second century saints made it clear that the Old Testament scriptures belong to—and were written for—the Church.
Valentinus (A.D. 150–160)
After failing to be appointed bishop in the city of Rome, Valentinus started a Gnostic sect, teaching against the catholic Church. Condemned by both Irenaeus and Tertullian of Carthage, his interpretations—and not necessarily his canon—of scripture was most problematic. While we don’t know his exact ‘list’ of books, the orthodox response to his teaching shows that whatever books he abused were accepted by the faithful at that time (with the exception of his Gnostic Gospel of Truth).
St. Irenaeus of Lyons (A.D. 130–200)
While he never set forth an exact canon of scripture, we do get the name ‘New Testament’ from this saint. Throughout his writings, Irenaeus cites every one of the twenty-seven books of the New Testament, with the exception of Hebrews, James, Jude and 2 Peter. He also cites both the Shepherd of Hermas and 1 Clement as scripture. Making reference to a sort of canon, he writes: “the entire Scriptures, the prophets and the Gospels” (Against Heresies 2.27.2). This is similar to Justin Martyr’s description of the Old Testament writings alongside “the memoirs of the apostles” in a second century liturgy.
Tertullian of Carthage (A.D. 160–225)
Tertullian also refers to the four Gospels as Christian scripture: “Of the apostles, therefore, John and Matthew first [instill] the faith into us; whilst of apostolic men, Luke and Mark [renew] it afterwards. These all start with the same principles of faith” (Against Marcion 4.2.2). Before his apostasy, Tertullian referenced as scripture the four Gospels, thirteen of Paul’s letters (excluding Hebrews), Acts, 1 John, 1 Peter, Jude, and Revelation. Like Irenaeus and others, the Shepherd of Hermas was frequently cited by him as authoritative.
Clement of Alexandria (A.D. 150–215)
Clement, though not a saint, is an early witness to a rather broad approach to the canon of New Testament scripture. Throughout his writings, he cites as authoritative the entirety of the New Testament, with the exception of James, 2 Peter, and 3 John. He also references the Epistle of Barnabas, the Shepherd of Hermas, 1 Clement, the Preaching of Peter, the Sibylline oracles, and the Didache. Per Eusebius, Clement is an early witness to Paul’s authorship of Hebrews, although he reports that Luke translated the original from Aramaic into Greek.
Eusebius of Caesarea (A.D. 325–330)
Eusebius is one of the first Christians to propose a closed canon of New Testament scriptures. In Ecclesiastical History, he references three categories of writings: canonical, disputed, and spurious. This he possibly received from Origen. As canonical, Eusebius mentions the four Gospels, Acts, the fourteen epistles of Paul, 1 Peter, 1 John, and Revelation. The disputed were James, Jude, 2 Peter, and 2–3 John. The spurious books were the Acts of Paul, the Shepherd of Hermas, the Apocalypse of Peter, and the Didache. In other words, by the mid-fourth century, at least twenty of the current twenty-seven books of the New Testament were universally received (at least, according to Eusebius). On the other hand, this meant that even by the fourth century, no consensus on what constituted the ‘Bible’ had been reached.
The Muratorian Fragment (A.D. 350–400)
Often (wrongly) referenced as a second century writing, this fragment contains an enumeration of New Testament scriptures. Likely written in the East after the lifetime of Eusebius,1 this fragment refers to the four Gospels, Paul’s letters to the Corinthians (two), Ephesians, Philippians, Colossians, Galatians, Thessalonians (two), and Romans, as well as letters to Philemon, Titus, and Timothy (two). The Apocalypses of both John and Peter, Jude, and two of John’s letters are also mentioned. The Shepherd of Hermas is included, with a note that “it cannot be read publicly in the Church to the people.” The Wisdom of Solomon is also included as “accepted in the catholic Church,” and written in Solomon’s honor. A dispute over the Apocalypse of Peter is noted, “which some of our people do not want to have read in the Church.” Even if a doubtful, earlier date of this fragment is granted, it does not establish a canon of either the New Testament or the Bible as a whole as some might understand it today.
St. Athanasius of Alexandria (A.D. 367)
In an annual encyclical as sent by the Alexandrian See for the dating of Pascha (Easter), Athanasius first lists the same twenty-seven books as accepted today. While this matches our canon today, debates over the New Testament scriptures would continue in both the East and West for centuries.
St. Cyril of Jerusalem (A.D. 360–386)
In his Catechetical Lectures, St. Cyril enumerates the same list of New Testament scriptures as Athanasius, with the exception of Revelation. He also derides the Gospel of Thomas, saying it “corrupts the souls of the simpler ones.” Of other writings, he says: “let all the rest, however, be placed in secondary [rank].” This indicates the place of ‘secondary’ writings in the life of the Church, which are not to be either read in liturgy or placed on the same level as sacred scripture.
Following the principle of consensus, none of the above necessarily represents the beliefs of the entire Church at any point through the end of the fourth century. The ‘canonization’ of the New Testament was a local concern for centuries, never being the primary focus of an Ecumenical Council. As with other matters of liturgy, each bishop had authority in the life of their church to rule on which scriptures were to be read (as with St. Athanasius in 367).
When finally noted by the Sixth Ecumenical Council (at the end of the seventh century), many different canons of scripture from previous, local synods were affirmed. Between East and West, the twenty-seven books of the New Testament have come to be universally recognized, while most of the Reformation debates on this issue dealt with the canon of the Old Testament.