How can we know that the Torah was given by Hashem and not some man or woman wrote and passed it on? The answer to this important and legitimate question is found in the Parasha!
In this pasuk Hashem commands Moshe to tell Bnei Israel to prepare because on the third day he will reveal himself to ALL THE PEOPLE.
While in other religions "God" only speaks to a single person, who then passes on the belief in God and His laws to other people and imposes them by force, in our Torah Hashem speaks to the ENTIRE PEOPLE. This proves something very important to us, using an example to explain exactly what:
If a teacher at school tells his 20 students to tell their parents and their future children that he is God and all living people should obey his laws, there is a high probability that the teacher will be fired and his 20 students will be at most their friends and Children say what an idiot their teacher was, but never will the whole class be able to convey the message to those around them as if it were true that their teacher was God. The larger the group, the harder it is for the teacher to convince his students.
In order for a whole group of students to confidently tell their future children what the teacher said and educate their children with these laws, the teacher has to prove that he has something supernatural or is God.
This is exactly what Hashem proved to our ancestors and this is exactly why we have all this information, laws and stories today!
Our parents passed it on from their parents and theirs from their parents to the generations of Har Sinai! In front of Har Sinai stood millions of Jews, it must be 100% something unbelievable and unnatural
Har Sinai must have happened so that so many passed on the Torah! If nothing blatant had ever happened before Har Sinai, so many people would never have passed on the Torah!
The generation of Har Sinai is not even behind us as long as we think. According to rabbinic calculations, Har Sinai was in the year 2448, which is 3334 years ago. Sounds like an eternity, but if you consider that it takes an average of 25 years from the time you are born until you have children (i.e. a generation passes), it is approx. 134 generations from Har Sinai to today. Each of us knows at least three generations, example: you, your mother, your mother's father. This effectively means that it only takes you, one of your parents and one of your grandparents 44 times to get to the generation of Har Sinai! The transmission of the Torah is completely realistic in terms of time over these 44 parts.
So in short, we know that the Torah was given by Hashem because he proved himself before the whole Jewish generation of har Sinai, that generation passed the Torah because they were proved that the Torah is divine. If Hashem had not proven himself in front of everyone and worked miracles, our ancestors would never have believed in the Torah and would never have passed it on! We have the Torah today so it must be from Hashem too!
The Torah is a work of Hashem, we should always remember that the Torah was given by him and is not man-made!