The twenty-seven books that make up the New Testament appear in the 39th Festal Letter of Athanasius (c. 367)—a prominent bishop—and this is the typical starting point for many regarding their history of the canon. Because of such a detailed letter regarding the New Testament, some have concluded that the canon was a late invention considering the letter’s dating. Still, the canon emerged in the first century and is evident in the writings of the early church fathers as functional before the fourth century. Athanasius wrote this letter to end the disputes about other orthodox letters believed to be equal to apostolic writings—Shepherd of Hermas and Epistle of Barnabas. It also detailed Peter’s epistles and Revelation’s authenticity, which was questionable to some.
Earlier than his festal letter, a catalog from North Africa listed twenty-four books named the Mommsen Catalog (c. 359). Cyril of Jerusalem had earlier listed all the books except Revelation (c. 350), but the Council of Nicea is often the canon’s accepted settling point (c. 325). However, the council’s entirety wasn’t about the canon, but the divinity of Jesus, hence the Nicene creed. Because they affirmed the canon doesn’t mean they “created” it. Similarly, regional church councils acknowledged the canon but didn’t determine it. As Michael Kruger puts it, “These councils were declaring the way things had been, not the way they wanted them to be.”
Around 250 CE, the early church theologian, Origen produced a list of the New Testament in his commentary on Joshua.
Matthew … Mark also; Luke and John each … Even Peter … in two of his epistles; also James and Jude. In addition, John … through his epistles, and Luke, as he describes the Acts of the Apostles … in fourteen of [Paul’s] epistles. (Hom. Josh. 7:1; cf. Hom. Gen. 13.2)
This list would have included Revelation and John’s epistles; Hebrews would have counted as a letter of Paul because some in the early church believed that Paul wrote Hebrews.
What, however, precipitated the list and the official declaration of the canon? It all began around 144 CE because of an early church heretic, Marcion. He only listed the gospel of Luke and ten of Paul’s epistles, omitting the pastorals and Hebrews. A harmony of the four gospels appeared in 170 CE by Tatian and was entitled Diatessaron. At about the same time, Melito of Sardis identified the Old Testament canon used by the Jews. The earliest response to Marcion’s list with list is the Muratorian Canon (c. 180), named after its discoverer. It contains twenty-two of our twenty-seven books, omitting James, 1 & 2 Peter, 3 John, and Hebrews. Interestingly, around the same time as the Muratorian Canon, Irenaeus was the first to refer to a New Testament (Adv. Haer. 4.15.2).
This leads to the truth that some books we now acknowledge as inspired weren’t always regarded as such by everyone in the early church. Athanasius explained the reasoning for excluding two well-regarded writings, but a few were disputed earlier, such as 2 Peter, 2–3 John, Jude, and James. Other writings were outright rejected: Acts of Paul, Apocalypse of Peter, Gospels of Peter, Thomas, and Matthias (Eus. Eccl. Hist. 3.25).