Why does the Catholic Bible have 73 books when Protestant Bibles have 66?

The seven books in question are Tobit, Judith, Wisdom, Sirach, Baruch, 1 and 2 Maccabees, plus portions of Esther and Daniel. Catholics call them the deuterocanonical books; Protestants call them the Apocrypha.

They were in the Bible the apostles used. The Septuagint, the Greek Old Testament, included them, and the New Testament quotes the Septuagint about 300 times. Hebrews 11:35 alludes to 2 Maccabees 7. The early Christians read these books as Scripture.

The Church settled the canon early. The Councils of Hippo (393) and Carthage (397), with St. Augustine present, listed the 73 books. Pope Innocent I confirmed it in 405. This is the same process that gave us the 27-book New Testament; there was no separate list from heaven. The Council of Trent solemnly defined the canon in 1546 in response to the Reformers, but it was reaffirming what the Church had held for over a thousand years (CCC 120).

Why were they removed? Martin Luther questioned them, in part because 2 Maccabees 12:44-46 supports prayer for the dead, which undergirds the doctrine of purgatory. He also wanted to drop James, Hebrews, Jude, and Revelation from the New Testament, but was overruled. Protestant Bibles kept the Apocrypha in a separate section until the nineteenth century; the King James Version of 1611 included them.

The Jewish canon argument. Some say the Church should follow the Jewish canon fixed at Jamnia (c. 90 AD). But that was a decision of rabbis who had rejected Christ, made after the Church already existed, and there is doubt Jamnia fixed anything. The Church, guided by the Spirit into all truth (Jn 16:13), determines the canon, and she did so with 73 books.