Why do Roman Catholic and Eastern Orthodox Bibles include more Old Testament books than Protestant Bibles?

The difference in the number of Old Testament books between Christian traditions stems from their historical reliance on different source texts and the subsequent decisions made during the Reformation.

Early Christian churches primarily used the Septuagint, a Greek translation of the Hebrew Bible produced in the 3rd and 2nd centuries BCE. This version included several books and expanded texts not found in the Hebrew canon, which were later categorized as apocrypha or deuterocanonical works.

During the Reformation, Protestant leaders sought to return to the Hebrew canon used by Jewish scholars, leading them to exclude these additional books from their Bibles or relegate them to a separate section. In contrast, the Roman Catholic Church, through the Council of Trent, formally affirmed the inclusion of these deuterocanonical books as canonical scripture. Eastern Orthodox churches also maintain a broader canon derived from the Septuagint, though their specific selection of books differs slightly from the Roman Catholic list.12