R. Jonathan Sacks, Rabbi Jonathan Sacks's Haggah, haggadah section p. 42f.:
If He had brought us to Mount Sinai and not given us the Torah
Maimonides writes (Hilkhot Yesodei ha-Torah 8:1) that the Israelites did not believe in Moses because of the miracles he performed. Miracles always leave open the possibility of scepticism: perhaps it was magic, illusion, chance or luck. Even when the Israelites 'believed in God and in Moses His servant' at the Red Sea, their belief was only temporary. What persuaded them of the truth of Moses and his prophecy was the revelation of God on Mount Sinai -- the only occasion in history in which God appeared, not to a prophet or holy person but to an entire people. Thus, had God appeared at Sinai, even without giving us the Torah, it would still have established Jewish faith in the existence of God an His involvement in history on the basis of the direct experience of an entire nation, not the testimony of a prophet.