Moshe Rabbeinu tries to give a proof that Judaism is the one and only true religion. He bases this on the Pasuk, או הנסה אלוקים לבוא לקחת לו גוי מקרב גוי, “Has G-d ever attempted to take one nation from the midst of another nation?”
Moshe is explaining the origin of Judaism. It began with our miraculous exodus from the most powerful nation on earth. This was witnessed by signs and wonders that were never seen before.
Can any other religion make such a claim in terms of how their religion began? Was it witnessed by millions of people with supernatural miracles coming from Hashem?
Only Judaism can make such a claim. Rav Yehuda Halevi in Kuzari, adds that the fact that these events are not disputed by the nations of the world, attests to the truth of these events.
Moshe is reminding the Jewish people. There is only one true religion. And that is Judaism.