To add a bit from the Kuzari:
1. In general fathers do not lie to their children *about important, especially positive events in their family history* (i added a lot the kuzari didnt say all that, just trying to deflect some objections to that statement.)
2. this is tricky, you have to think about it:
people will not believe that an event took place if it was an event such that it should have left a great deal of evidence, but it didnt
Let me give an example. you would not believe that a large meteor struck new york city 100 years ago, destroying most of it, then it was rebuilt. why? the event should have been well remembered by people living in that generation who told it to others, and their children, many stories about it , thus becoming history.
the Jewish People, perhaps the most cynical and stubborn peoples to have ever existed, would certainly never have swallowed the tale of Har Sinai at any point in their existence, had it not happened, because there would have been no evidence or history or remembrances of it had it not happened.
They would never have accepted:
Uh guys, well my father told me that Moses told hm that all of our fathers received this Torah a while back. Why haven’t you heard about it? I’ll explain that later.
Why should you accept it?
Well, even though you never heard of it before there is a lot of good things in it i’m sure you will like, and will gladly accept even though you are still a little skeptical.
Like what?
Well like 3 times a year you have to leave your fields, your property, your wives, your little children and travel up to Jerusalem. Dont worry Im sure our friendly neighbors won’t hear about this and if they did they wouldnt do anything.
Oh yes, and I have a huge list of all kinds of taxes you have to pay to the Kohanim, so they don’t have to work like you.
Lets see, oh yes you cant eat this and this and this, and every seven years you cant work your fields at all (not a portion but the entire field)
oh I almost forgot, not just the Kohanim. Theres a heavy welfare system for the poor also
And once a week you cant work, of course if you really want to you can, but then well have to kill you.
This is just the beginning folks, but dont worry, after you die youll get rewarded for all this