The original Hebrew Old Testament includes 39 books. But in approximately 200 BC, when Hebrew scholars translated the bible into Greek, the texts expanded to include 46 books. Hellenistic Jews used this translation, called the Septuagint, as did many early Christians.
In approximately 382 AD, the ascetic St. Jerome decided to translate the original Hebrew bible into Latin. During this process, St. Jerome became convinced that the Septuagint included inauthentic books. Another ascetic and one of the period’s most prolific scholars, St. Augustine, began writing to Jerome and defending the holiness of the Greek translation. The two saints never reached an agreement and neither did their successors.