Moshe Rabbeinu warns the Jewish people that they should be careful not to get complacent when living in the Land. He wanted them to remember the special events that they had witnessed when they became a nation.
Specifically, Moshe was referring to what was seen on Mount Sinai and Egypt. There is a Pasuk that follows that shows how Judaism is the only true religion. The Pasuk says, או הניסה אלוקים לקחת לו גוי מקרב גוי. Can any nation make the same claim as the Jewish people? Was there ever a people who were taken out from the midst of another nation with signs and wonders as the Jews were?
All other religions began with a leader and a handful of disciples claiming to have been ordained by G-d. The Jewish religion began with the entire nation witnessing great miracles that culminated with all of the people hearing Hashem speak.
The Kuzari says that what makes this proof even more convincing is the fact that the nations of the world accept that these events actually did happen.
The Torah itself shows that only Judaism is true and the other religions are false.