Story
How the New Testament canon formed
A slow argument about which books to read in church, not a single vote that invented the Bible.
Step 1
Letters already circulating
By the early second century, Paul's letters were being collected and read beyond the cities that first received them. 1 Clement and Ignatius already treat some Christian writings as authoritative, without publishing a closed list.
Step 2
Four gospels, and rivals
Irenaeus, around 180, insists on four gospels and no others. He is arguing, which means the limit was not yet obvious to everyone. Marcion had already forced the question by publishing his own shorter gospel and Paul collection.
Step 3
Useful, disputed, and rejected
Third-century writers such as Origen distinguish books everyone uses, books some churches dispute (Hebrews, Revelation, 2 Peter, and others, depending on the region), and books they reject. The Shepherd of Hermas was loved and still often kept out of the public canon.
Step 4
Lists harden in the fourth century
Athanasius's Festal Letter 39 (367) is the first known list of exactly the 27 New Testament books. African councils at Hippo (393) and Carthage (397) name a similar set. The Muratorian fragment is an earlier Latin list, but scholars dispute whether it is second-century or fourth-century. Eastern practice took longer to line up, especially on Revelation.