Who decided which books went into the Bible?

Nobody decided it in one go. The Bible's list of books, its canon (from the Greek kanōn, a measuring stick), grew over centuries of use and debate, and different communities still draw it differently. The Jewish Tanakh has 24 books. Protestant Bibles divide the same material into 39 Old Testament books; Catholic Bibles have 46, adding the deuterocanonical books. Most churches agree on 27 books in the New Testament, though Orthodox and Ethiopian churches have some differences.

For the Christian New Testament, a provocative teacher forced the question. Around 130–140 CE, Marcion of Sinope drew up what may be the first definitive list: ten letters of Paul and an edited gospel, rejecting the Old Testament entirely. The church condemned him, but now it had to say which books counted and why. By about 200, a list known as the Muratorian fragment included four gospels. In 367, Bishop Athanasius of Alexandria listed exactly the 27 books of today's New Testament. Revelation was the last to win universal acceptance.

A popular myth says the Council of Nicaea in 325 chose the books. There is no record of any discussion of the canon there. The story owes much to Voltaire, who popularized a tale of books placed on an altar, with the ones that didn't fall off kept as scripture.

The debate even resurfaced later: Martin Luther pushed four books to the back of his Bible, and the Council of Trent replied in 1546 by fixing the Catholic list.