Who decided which books are in the Bible?

The Bible did not come with a table of contents

No book of Scripture lists which other books belong. The "canon" (from Greek kanōn, "measuring rule") had to be recognized. The Catechism says: "It was by the apostolic Tradition that the Church discerned which writings are to be included in the list of the sacred books" (CCC 120).

How the Church discerned

Three criteria guided the early Church:

Apostolic origin — written by an apostle or a close associate.
Orthodoxy — consistent with the faith handed down.
Liturgical use — read publicly at the Eucharist across the churches.

Many writings were rejected, including the Gospel of Thomas and the Gospel of Peter.

Key dates

c. 367 — Saint Athanasius lists the 27 New Testament books.
382 — A synod in Rome under Pope Damasus is traditionally associated with a list of the canon (scholars debate the details of the surviving document).
393 and 397 — Councils of Hippo and Carthage list the 73 books Catholics use today.
1442 — The Council of Florence affirms the same list.
1546 — The Council of Trent solemnly defines the canon after it was challenged in the Reformation.

Why it matters

Christians who accept the 27 New Testament books are already relying on the discernment of the early Church. Catholics see this as the work of the same Holy Spirit who inspired the authors, guiding the Church to recognize them.