How do we know that the revelation at Sinai was not a magician's trick?

Question

As quoted in this answer:

On the fiftieth day after they began counting the Omer--that is, fifty-one days after the Exodus, all of the Children of Israel, men, women and children, over two million people, stood at Mount Sinai and received the Torah amidst great miracles and heavenly fire. They saw no form or picture of G-d, but they saw many miracles that proved that G-d is the Creator of heaven and earth. They heard G-d's voice speak and command Moses to instruct the Children of Israel on how to prepare to receive the Torah. Then they heard G-d speaking directly to them, the Children of Israel, and commanding them to keep the Torah. The Children of Israel accepted the Torah and all its Commandments, and they said: "We agree to obey, even before we hear the actual Commandments."

We are told not to believe in miracles; how do we know that the voice of God at Sinai wasn't some magician's ploy to convert the masses and how do we know that the miracles that took place at Sinai were done by God and not some magician?

Answer 1

Two approaches:

The Unmistakable Nature of Divine Experience

The Jewish people themselves heard Hashem speak at Har Sinai, on some level of prophecy. The Ramchal writes in Derech Hashem 3:3:3 that an experience of revelation, on whatever level, is unmistakable:

אמנם בכלן תהיה ההשפעה בדרך שירגיש בה המשפע בברור

In all of [the levels of revelation] the spiritual influence comes in a way in which it is clear to the recipient.

So if they heard it themselves, they would know unmistakably that it was a true Revelation - authentic revelation cannot be faked.

The Nature of Direct and Indirect Proof

The Malbim's commentary to Shemos 19:9 cites the Sefer HaIkarim's explanation of this distinction with an analogy. If two people say they can make triak (an ancient mythical panacea), and the first shows you as he makes it that he has all of the ingredients and lets you watch as he puts them together. Or, he makes it and then tests it and you see that it is effective. The other person does all kinds of wonderous things like walks through fire and the like, and tells you that as he can do wonderous things, he clearly has some power and you should believe him. In other words, the second person is asking you to transfer your belief from his wonders to his claims. The same is true of any prophet viz-a-viz Moshe. Moshe did not do wonders and, based thereon, ask us to rely on him. He showed us that Hashem speaks to him. There is no request for us to transfer our belief from him to his claims. This direct belief is what distinguishes Moshe from any other prophet, and it is these kinds of indirect beliefs which are subject to doubt.

In this type of demonstration, the doubt would not be on Moshe's word, but rather on the reliability of our own experiences. This is the superiority of demonstration which is present in Moshe's credentials as messenger from Hashem.

Answer 2

In order to pose such a possibility we would need an example of magic being done in such magnitude. The voice of Hashem was heard by all. It's source couldn't be traced to any one side, according to the Medrash. The mountain was smoking and yet Moshe Rabbeinu stood there, and Hashem spoke before he even got there.

This was after the plagues and splitting of the sea. Following this was the precise Hashgacha Pratis in the wilderness, where Korach, Dassan and Aviram died for going against the Torah. They had these large scale, foretold miracles that their lives depended on.

Less than a thousand years later was the Galus Bavel, where we ceased to be a unified people. And yet, we held to the Torah. I couldn't make up a large scale story about the main history of the crusades, the Spanish wars and unification and the Magna Carta because they are relatively recent history, within a thousand years.

There were no parallel Judaisms having their own story of origin, although there were two kingdoms in Israel from pretty early on, and after not having a king at all.

If you want to know what a man-made religion would look like you have examples to compare. Murky foundations, lots of self-praise, threatening those who didn't make the leap. The Torah does not have that.