The giving of the Torah at Mount Sinai was the single most important event in the history of the Jews. It was more important than the creation of the world, and more important than the building or the destruction of the Temples in Jerusalem. The giving of the Torah defines us as a nation and provides us with a path through our lives.
What happened at Mount Sinai is also the foundation of our belief. That is because the creation of the world isn't independently verifiable as there was no living creature there to see it. The existence of a Jewish nation doesn't prove anything either as there have been many nations in the history of the world. Their existence proved nothing about G-d. But the claim that G-d took two million or more people out of slavery in Egypt, brought them to a small mountain in the Sinai desert, appeared to them, spoke to them with an audible voice, and then gave them 613 commandments through a divinely appointed messenger (Moses) - that's a claim which cannot be ignored.
Rabbi Boruch Clinton teaches at the Ottawa Torah Institute yeshiva high
school and Machon Sarah high school for girls (both in Ottawa, Canada).
You may reach him with comments and questions at