Thank you for the question on the divisions of the Bible, “Why wasn’t the Bible separated into more than just the Old Testament and the New Testament?”
As one reads the Old Testament and into the Gospels, how did the Jews viewed the Scriptures? One must remember that in the Old Testament, there weren’t one singular manuscript of all the Old Testament books. The Jewish people viewed the Scriptures into three parts: The Torah, The Prophets which included the Former Prophets and the Latter Prophets, and the Writings. The Old Testament scrolls/books and their listing under the main divisions are slightly different from the Christian ordering of the books. One can read about this on this website: Encyclopedia Britannica. The Jews saw the Scriptures in a three fold division: The Torah, The Prophets, and the Writings.
Jesus implies that the Scriptures were in three fold in Luke 24. Jesus said this, “He said to them, “How foolish you are, and how slow to believe all that the prophets have spoken! 26 Did not the Messiah have to suffer these things and then enter his glory?” 27 And beginning with Moses and all the Prophets, he explained to them what was said in all the Scriptures concerning himself” (A searchable online Bible in over 150 versions and 50 languages).
Jesus referred to Moses or the Torah, all the Prophets, and in all the Scriptures or the Writings. Since Jesus already referred to Moses, the Prophets, the only remaining division would be the Writings. The combining of the three fold divisions were known as the Scriptures. As one may know the New Testament is also divided into three folds with the Gospels, the Historical Book (Acts) and the Epistles (Pauline Epistles and the General Epistles).
Having shown the three fold divisions of the Old Testament and New Testament, the reasons for upholding the two fold division are as follows:
A. Separation of the Bible into more than two main divisions would require more effort to memorize the divisions and the books within those division.
B. Separation of the Bible into more than two divisions would refocus the central person who bridges both the Old and New Testament, Jesus Christ.
C. Separation of the Bible into more than two divisions may cause greater questioning of the canonicity of the Scriptures. It is easier to explain two main divisions of the Bible than 3 or 4 divisions and the coordination of the Bible.
D. Separation of the Bible into more than two divisions allows for the greater contrast between Law and Grace, the Old Covenant and the New Covenant, the nation of Israel and the world’s people of the Church.
There is nothing magical about calling the Books of the Jews as Old Testament and the letters of the Apostles as New Testament. It is a designation that helps believers understand the Old Testament refers to the Jewish people and the New Testament refers to the Christians.
SUMMARY: The classification of the Old Testament and the New Testament provided the clearest contrast between the teachings of the Law and the teachings of Grace.
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