This is the idiotic claim of the Catholic Church. How many times in scripture does it tell us, "It is written...". Our Lord Jesus himself picked up a scroll and READ from Isaiah.
So the Catholics tell us everything was handed down orally and those spritures that were handed down orally tells us they were written. It would almost be laughable if it wasn't so sad.
A large portion of the nt was written by people who never met Jesus that is my point. At least Luke and Paul were never present for any of his teaching. Therefore most of it was oral tradition which is unreliable by protestant standards.