A list of the works to be considered authoritative began to take form in the Christian community earlier than I had supposed. We always hear that the “definitive” list was not compiled in the 4th century, but less attention is paid to the fact that many of these works were considered authoritative well before then.
In about 180 CE (at almost the same time Irenaeus was defending the four Gospels against Marcion, who wanted to acknowledge Luke alone) there appeared the first listing of the books of the New Testament that bears a similarity with the present Christian canon. The Muratorian canon (as the list came to be known) listed the four Gospels, the Acts of the Apostles, the Pauline letters, and the rest of the present New Testament with the omission of Hebrews, James, and the two letters attributed to Peter. The list included, however, the Shepard of Hermas, a popular work of the late first century. No definitive canon was established until the fourth century, and even then there would be disagreement over the Epistle to the Hebrews, but the attempt to form a distinctive Christian canon had begun.
– Robert Bruce Mullin, A Short World History of Christianity (Westminster John Knox Press, 2008)