The underlying truth is the rules for living together with your neighbors. Jesus spoke in parables, which were both true and fictional.
So, they lied about a guy named Moses meeting God, they lied about God giving him tablets, they lied about God writing commandments on those tablets, and they lied about this happening on Mt Sinai...
BUT.....
The moral code introduced by all this fiction is true?
To be honest with you, there is nothing provably true within that code.
It really doesn't make any difference if I steal your wife, your money, and your donkey.
If I get away with it, tough luck for you.
If I don't, tough luck for me. The world goes on.
Those rules don't mean sh_t unless God gave them.