When Moses went up to heaven to receive the Torah, God gave him the Written Torah together with many instructions. These instructions are called “Halachah L’Moshe M’Sinai” (the Law that was given to Moses on Sinai). Maimonides writes that it is impossible for there to be an argument or disagreement concerning a Halachah L’Moshe M’Sinai, for the Jews who heard the instructions from Moses implemented them into their daily lives and passed it on to their children, who passed it on to their children, etc.
Deuteronomy 4 tells us that Moses told the people of Israel to hear the statutes and decrees that he was teaching them to observe, and he concluded:
“However, take care and be earnestly on your guard
not to forget the things which your own eyes have seen,
nor let them slip from your memory as long as you live,
but teach them to your children and to your children’s children.”
I can imagine that when the people took possession of the land that the Lord gave them, they were very careful to observe the statutes and decrees that God commanded to Moses, that they committed them to memory, that they had their children and their children’s children commit them to memory, and that their children and their children’s children did the same until they were codified into the Torah.
Acts 2 tells us that the first Christians devoted themselves to the teaching of the apostles and to the communal life, and every day they devoted themselves to meeting together in the temple area and to breaking bread in their homes.
I can equally imagine that these Jewish Christians likewise were very careful to observe the teachings that Jesus commanded to the apostles, that they committed them to memory, , that they had their children and their children’s children commit them to memory, and that their children and their children’s children did the same until they were codified into the New Testament and the Sacred Tradition of the Church.
And it wasn’t just a few who took the time to memorize and study it. It was pretty much everyone.
“I can imagine that when the people took possession of the land that the Lord gave them, they were very careful to observe the statutes and decrees that God commanded to Moses, that they committed them to memory, that they had their children and their children’s children commit them to memory, and that their children and their children’s children did the same until they were codified into the Torah.”
You might imagine that, but I read a different story in the Old Testament, where the Israelites were constantly backsliding, and at least once, seem to have wholly abandoned the law. At that point, when King Josiah rediscovered a copy of the Torah in the Ark of the Covenant (2 Kings 22-23) he read it and rent his clothes in grief over how the people had sinned against God. Then he assembled all the people in the Temple and made them reaffirm the covenant they had forsaken.
So, I doubt if the oral tradition was faithfully kept, when the Bible attests that even the written tradition was forgotten.
No doubt a lot of memorization was going on in following generations. However, we do have affirmations of the following which shows us what God said was Written:
Exodus 31:18 KJV
And he gave unto Moses, when he had made an end of communing with him upon mount Sinai, two tables of testimony, tables of stone, written with the finger of God.(KJV)
And once again under Joshua:
Joshua 24:25-26 KJV
So Joshua made a covenant with the people that day, and set them a statute and an ordinance in Shechem. And Joshua wrote these words in the book of the law of God, and took a great stone, and set it up there under an oak, that was by the sanctuary of the Lord.(KJV)
Joshua 8:31-32 KJV
As Moses the servant of the Lord commanded the children of Israel, as it is written in the book of the law of Moses, an altar of whole stones, over which no man hath lift up any iron: and they offered thereon burnt offerings unto the Lord , and sacrificed peace offerings. And he wrote there upon the stones a copy of the law of Moses, which he wrote in the presence of the children of Israel.(KJV)
Here also:
Deuteronomy 31:9 KJV
And Moses wrote this law, and delivered it unto the priests the sons of Levi, which bare the ark of the covenant of the Lord , and unto all the elders of Israel.(KJV)
And for Christians? Jesus Christ left no doubt. He did not say “it is said” or “tradition says” but “it is written.”
Matthew 4:4, 7, 10 KJV
But he answered and said, It is written, Man shall not live by bread alone, but by every word that proceedeth out of the mouth of God. Jesus said unto him,
It is written again, Thou shalt not tempt the Lord thy God.
Then saith Jesus unto him, Get thee hence, Satan: for it is written, Thou shalt worship the Lord thy God, and him only shalt thou serve.