Around 200 BC a group of Hebrew scholars met in Alexandria to translate the Old Testament into Greek. The translation is referred to as the Spetuagint (LXX) and is quoted extensively (about 300 times) in the New Testament. The Septuagint listed 46 books in the Old Testament, and it is this canon (official list) which is accepted in the Roman Catholic traditon.
Around 90 AD a group of Hebrew scholars met in Jamnia to determine the canon of the Old Testament. Only those works which were written in the Hebrew language and within the territorial confines of Palestine were accepted--a total of 39 books. It is this canon which is accepted in the Protestant tradition.
Which collection is authentically God's Word?
The criteria established at Jamnia need to be founded upon some revelation of God--is there such revelation?--so it seems, at least to be consistent with the Protestant notion of Revelation. For why would there need to be a council of Hebrew scholars to determine what comprises the official list of Scripture if what God wants made known automatically gets into the Bible anyway?
In the famous story of the Ethiopian eunuch riding in his chariot as decribed in the Acts of the Apostles, while reading from the Book of Isaiah, he was asked by one of the Apostles, Philip, if he understood what he was reading.
The point here is that the Catholic Tradition is built upon a living organism, the Mystical Body of Jesus Christ, which rests not on texts alone, but which acts much as does the soul upon the human body.
It is this living tradition that allows the Church to recognize the Septuagint as recording God's revelation, even to the cusp of New Testament times, in a foreign language as a precursor, possibly, of the invitation to the Gentile world that the Good News is for it too.
The Catholic Church lived the Faith given by Jesus to the Apostles before the Church recorded what is found in the Bible. In like manner, the Church lives today in constant meditation upon Sacred Scripture but the Church also lives in constant meditation upon her history, her natural gifts, and the spiritual wisdom which has come with lived experience.
There is no evidence to suggest that God has only revealed Himself via the written word. As a matter of history, the opposite is true.
Biblical Index to the Teachings of Jesus
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