I. Is there really a mesora about matan torah?
If you listen to anyone who tries to prove that Judaism is true, their main argument is the so called "Kuzari Proof". The essence of the "Kuzari Proof" is as follows. No one could invent a story that alleges all of the target’s audience’s ancestors experienced some miraculous or otherwise special event. It is implausible that a lie could be consistently retold by millions. And, the audience’s reaction would be one of disbelief, “Why does he know all about this event, and we never heard of it before from our grandparents?”Since Judaism uniquely makes claims of national miracles and national revelations, events with audiences of millions who are the ancestors of nearly all of the target audience (excepting geirim) this gives Judaism a unique claim to authenticity. The commonality of the story amongst so many and the acceptance of the story by their descendents is unique. (In contrast, Jesus’s alleged miracles were only said to be witnessed by at most the 500 attendees of the wedding at Cana, and the target audience isn’t primarily the descendents of those guests.)
The first refutation of the Kuzari proof is very simple. There have been times when the jewish people forgot the Torah. This is not me saying this, the Ramban al Hatorah says this (במדבר ט"ו:כ"ב) when talking about how the entire Jewish people could sin בשוגג. He writes:
In our sinfulness, this has already happened in the days of the evil kings of Israel, such as Jeroboam, that most of the nation completely forgot Torah and the commandments, and the instance in the book of Ezra about the people of the Second Temple.
The Ramban writes that in the times of the first Beis Hamikdash as well as the time of Ezra most of the Jewish people completely forgot the Torah. In other words, most of the people had no mesora, no memory of matan torah and Ezra convinced them of the truth of Torah.
The assumption is made that the claim is made out of the blue, in a single stroke. It doesn’t account for gradual acceptance of a story. Say something starts out as a myth about a subset of the people, and it’s known to be a bed-time story. The next generation it’s “some say”. Over several generations, it can become “official history” about everyone, with no one generation expressing the disbelief that is critical to this argument. We see this happen all the time and this in fact one of the defining characteristics of an oral tradition.
II. Is there a Mesora about what happened at Har Sinai?
Since Matan Torah is the seminal event in Jewish and world history you would think that there would be a clear and undisputed tradition about the event. Yet, we find that disputes abound with respect to important aspects of what happened.
What date was the Torah given?
The average orthodox Jew will answer the 6th of Sivan. However, this is actually a machlokes in the Gemara Shabbos 86b:
Our Rabbis taught: On the sixth day of the month [Siwan] were the Ten Commandments given to Israel. R. Jose maintained: On the seventh thereof.
The Gemara explains that the machlokes is whether there were 2 or 3 days of פרישה.
How can it be that there isn't one clear tradition of the date of Matan Torah? How can there be a dispute about how many days of פרישה there were? How come the 600,000 people didn't all remember the same date and pass it on to their children?
What did the Jewish people hear at Har Sinai?
Again, you would think that there would be a clear and undisputed tradition about what was heard. In fact, this is a machlokes the Rambam and the Ramban. The Rambam states that for the first 2 dibros the people heard noise but did not hear or understand the words. For the last 8, they heard nothing. The Ramban claims that for the first 2 they heard and understood the words and for the last 8 they heard noise but did not hear or understand the words.
Again, this is the seminal event witnessed by millions of people and yet we don't even know what they people heard.
What did the Jewish people receive at Har Sinai?
Rashid at the beginning of Parshas Behar asks the famous questions מה ענין שמיטה אצל הר סיני and answers that it is to teach us that we received all of the details of all the mitzvos at Har Sinai. However, this is not so simple. In fact, there is a fundamental dispute between the Rambam and the Geonim. The Geonim state that Moshe got everything at Har Sinai and any later disputes are because things were forgotten. The Rambam vehemently disagrees and claims that Moshe got the principle of how to derive halachos plus the basic halachos and some details. However, he did NOT get every detail and machlokes is regarding things learned from the principles.
In short, there is a fundamental dispute about what was received at Har Sinai, everything or not (I am planning on devoting a whole essay to the development of disputes).