I have ripped most of this from “Why Do Catholics Do That?” by Kevin Orlin Johnson.
The first of Judaism’s sacred books (Gn, Ex, Lv, Nm, Dt) were written about 1250 B.C. These are known as the Torah and are the basic scripture for the Israelites. As the Jews formed their national identity and God revealed more of his nature and their mission, they wrote more and more books to explain these developments. That is how we got Js, Jdg, Kgs, and Chr (which used to be called Paralipomenon, Greek for “things left out” of kings). That’s why the prophecies of Ez, Neh, and others were written
But the Israelites wrote lots of other books, too. Such as the Words of the Days of the Kings of Israel and the parallel books for Juda mentioned in Kings. They wrote commentaries on every little point of every Law of Moses and opinions and sayings o great teachers, educational books using the name of some long-dead prophet who might have said (or ought to have said) such things. By 200 B.C. it got hard to tell where the books left off and the commentaries began.
Help came from a very unexpected quarter: the Pharaoh of Egypt, Ptolemy (the Greek speaking successor to Alexander the Great). He decided he wanted his library to have a copy of every single book in the world, all in alphabetical order. The problem was that he couldn’t make much sense of the mountains of Jewish writing and the covenant – like many of the Jews themselves. He commissioned seventy Jewish scholars to come up with a standard canon of scripture and a standard version of each book in the canon.
The 46 books the seventy (hence Septuagint) scholars came up with constituted the standard canon the was used right up to the time of Christ. These were the books that let his disciples recognize him (Jn 1:45) and reading them makes it very hard to deny that Christ is the Messiah. Every thing in these books points to that conclusion: not just the prophesies, but also the laws, poems, and even histories, which center around that one remarkable family from which he was born.
Outside the Church these books have had their share of tribulation. In the year 70 the Roman Emperor Titus led his armies into Jerusalem to put down a rebellion. They completely destroyed the Temple of Jerusalem and carried off the Ark of the Covenant and all of the necessary temple furnishings in triumph to Rome. You can still see them represented in the sculptures of the Arch of Titus.
The hard core of rabbis who still denied Jesus couldn’t perform the liturgies required by the Torah without the Temple. They undoubtedly thought the Ark would be brought back and the Temple rebuilt just like it had been dozens of times before. Their leader, Jochanan ben Zakkai, took them out to the village of Jamnia where they could assess the situation. They were determined to preserve the sacred trust as they understood it.
They need to develop a new form of Judaism that would unite all Jews, at least until the Temple could be rebuilt, and to undercut the Christian’s claims of the divinity of Jesus and his identity as the Christ. So, they assembled a new version of Jewish scripture, omitting some books entirely and rewriting others. This resulted in the Jamnian, or Palestinian canon, and it changed Judaism forever.
Protestant Reformers like Luther, Wycliff, Huss, and Calvin rejected certain tenants of Christianity. Of course, deletions or innovations are prevented by Sacred Tradition (which the Reformers therefore rejected entirely) and the New Testament (which they rephrased freely). But Christian teachings are also supported and clarified by the Old Testament. If the Reformers wanted to deny the importance of works in the Cycle of Redemption, they’d have to got rid of whole books like Tobit; to deny the existence of Purgatory, they’d have to dispose of Maccabees [1) You can find phrases in the New Testament that point to Purgatory, and 2) the Jewish Festival of Lights, Hanukkah, comes from the Maccabees]. Even then, they’d have to rephrase all of the remaining verses that echoed these teaching. That’s why the Jamnian canon appealed to them. The rabbis wrote it, after all, to remove the basis for a lot of the teachings that the Protestants were themselves rejecting. So the Reformers took the Septuagint out of their Bibles and substituted their own translations of the Jamnian canon.