Who decided which books belong in the Bible?

No single council sat down and picked books out of a hat. The New Testament's core—the four Gospels and Paul's letters—was already widely recognized and used across scattered, independent churches by the early second century, well before any formal list existed.

Early Christians generally asked three kinds of questions about a book: Did it come from an apostle or someone closely connected to one? Was it actually used and accepted broadly across churches, not just locally? Did it agree with the faith the church already held? Books like Hebrews, James, 2 Peter, and Revelation took longer to gain universal agreement—genuinely debated for a while.

A church leader named Athanasius listed all 27 New Testament books we have today in a letter in the year 367—the earliest such complete list to survive. Later regional councils largely confirmed what churches were already using, rather than creating something new by decree.

So "who decided" undersells it a little. It was less a single decision and more a gradual, cross-checked recognition, tested over generations by many different Christian communities, of which books already carried the weight of apostolic authority.