A Catholic Bible has seventy-three books in it while a typical Protestant Bible has sixty-six books. Both of them have the same number of books in the New Testament (twenty-seven); it is in the Old Testament that the two differ.
Catholic Bibles have seven more books (Judith, Ecclesiasticus or Sirach, Wisdom, Baruch, Maccabees 1 and 2, and Tobit) because they are using the older Greek (Alexandrian) list of authorized books, called a canon, going back to the year 250 BC. At that time, seventy scholars were commissioned to translate the thirty-nine already existing Old Testament books from Hebrew into Greek. They then included the seven other (called deuterocanonical) books which were originally written in Greek by Jews in Exile.
By the third century BC, two-thirds of the Jews were no longer in their homeland. The Babylonians and Assyrians had exiled a good portion of the Jewish people, and only a third were left behind. That small portion lived in what was then called Palestine (modern-day Israel), and they spoke and wrote in Hebrew. The majority of Jews who were in exile, however, were forbidden to teach their young the Hebrew language, so they grew up knowing Greek since it was the language of the Empire (of Alexander the Great), commerce, and academia.
Because those seventy scholars were alleged to have taken seventy days to translate the Hebrew scriptures into Greek, the Latin word for seventy (septuaginta) or the Roman numeral LXX was given to designate this collection of forty-six books (thirty-nine translated from Hebrew into Greek, plus seven originally written in Greek). Judaism and then Christianity accepted and used all forty-six books of the Septuagint until the year 90 AD, when the Jewish religious leaders decided to revise the list of authorized books.
After the Temple of Jerusalem was destroyed by the Romans in 70 AD, the Christians and Jews parted from each other and established separate religions. Twenty years later, as Jewish scholars became concerned that the Greek Christian influence had to be removed, the seven books of the Old Testament, which were never originally written in Hebrew but only in Greek, were dropped from the list. This newer Hebrew list is sometimes called the Palestinian Canon because of the name of the Holy Land at that time. Christians, however, now independent, retained their older Greek (Alexandrian) list of forty-six books, while the Jews kept only thirty-nine.
Saint Jerome was commissioned by Pope Damasus I in 400 AD to translate all the Hebrew and Greek texts of the Old and New Testaments into one language and one volume. At that time, Latin was the official language of the Empire. He used the Septuagint Greek version and retained all forty-six Old Testament books with the twenty-seven New Testament books to formulate the first single-volume edition of the Christian Bible, totaling seventy-three books.
Things didn’t change for fifteen centuries, until the Protestant Reformation. Martin Luther, an Augustinian monk, Catholic priest, and scripture scholar, knew the Jews had a shorter list of books (thirty-nine) compared to the longer list (forty-six), and he knew that some of the abuses and scandals in the late medieval church originated in a doctrine based on one of those deuterocanonical books (Maccabees, which is used to explain the Catholic doctrine of purgatory).
The sin of selling indulgences was too much for Luther to stomach, and he responded with a rejection not only of the abuse but also of the doctrine that the dead needed prayers to get to heaven. He embraced the Hebrew (Palestinian) canon of thirty-nine books in the Old Testament, and the newly translated German Bible that contained sixty-six books with no deuterocanonical books included at all. With Maccabees gone, Luther hoped the abuse would go as well.
Catholics, on the other hand, since the Council of Trent, decided to keep the Greek (Alexandrian) canon of forty-six books in the Old Testament, because that was what the Christians knew and used at the time of Christ and the apostles. It was also the same list used by Saint Jerome for his Vulgate Bible and has been consistently used ever since.
So since the time of the Reformation in the sixteenth century, there have been two kinds of the Bible: the Protestant and the Catholic, which are 99 percent identical in order and text except for those seven books written in the third century BC. Some Protestant Bibles now include them in a section called the Apocrypha, which comes at the end of the Old Testament before the Gospels of the New Testament. Catholic Bibles have always had these books, which they call deuterocanonical.
Both Bibles begin with Genesis and end with Revelation and have four Gospels. The only difference is the inclusion or exclusion of seven additional books in the Hebrew Scriptures (Christian Old Testament).