During the forty days he spent in heaven, Moses received beside the two tables all the Torah-the Bible, Mishnah, Talmud, and Haggadah, yea, even all that ever clever scholars would ask their teacher was revealed to him. When he now received the command from God to teach all this to Israel, he requested God to write down all the Torah and to give it to Israel in that way. But God said: "Gladly would I give them the whole in writing, but it is revealed before Me that the nations of the world will hereafter read the Torah translated into Greek, and will say: 'We are the true Israel, we are the children of God.'
Then I shall say to the nations: 'Ye claim to be MY children, do ye not know that those only are My children to whom I have confided My secret, the oral teaching?'" This was the reason why the Pentateuch only was given to Moses in writing, and the other parts of the Torah by word of mouth. Hence the covenant God made with Israel reads: "I gave ye a written and an oral Torah. My covenant with you says that ye shall study the written Torah as a written thing, and the oral as an oral; but in case you confound the one with the other you will not be rewarded.
For the Torah's sake alone have I made a covenant with you; had ye not accepted the Torah, I should not have acknowledged you before all other nations. Before you accepted the Torah, you were just like all other nations, and for the Torah's sake alone have I lifted you above the others. Even your king, Moses, owes the distinction he enjoys in this world and in the world hereafter to the Torah alone. Had you not accepted the Torah, then should I have dissolved the upper and the under worlds into chaos."