After the devastating description of the future troubles that will befall the people of Israel if they betray the Torah, Moshe again calls the people of Israel to address them: You have seen all that G-d did before your eyes in the land of Egypt, to Pharaoh and to all his servants and to all his land: The great acts of proof that your eyes have seen, those great signs and instructive miracles. But with all this, G-d did not give you a heart to recognize, or eyes to see, or ears to hear, to this day. Then, I led you for forty years in the wilderness; your clothes did not wear out on you, your shoes did not wear out on your feet. You neither ate bread nor drank wine… so that you might know that G-d is your G-d. (Devarim 29:3-5, Hirsch translation)
What is the difference between the impact of the experience of the miracles and the great proofs, on the one hand, and the forty years of supernatural sustenance, on the other? Rav Hirsch explains that the experience of miraculous exodus should have sufficed to give the people knowledge of G-d — of His power, His nature, His rule, His justice. This should have transformed their view of the world and made their ear receptive to truth and duty, making them fit to be the people of G-d in the Land of G‑d.
However, these experiences were still not enough to permanently impact their hearts, and allow the people to hear and see all the phenomena of the world as the handiwork of an all-powerful and ever-present G-d. Instead, it would require forty years of miraculous sustenance — the experience of having their personal needs, such as clothing, food and shelter, being cared for — to truly know that there is one G-d, exalted above all, yet near to all. The personal relevance of G-d to their lives could only be taught during these forty years of protection, trials and travails. For it is during these forty years that the nation learned that at every moment, G-d watches over and guides, examines and judges, helps and halts the fate and accomplishments of the nation and of every individual.
Rav Hirsch explains the verse in this way: “That G-d” — the one responsible for the Exodus and who guides the nation, is also “your G-d” — the G-d of each individual. This was the complete knowledge of G-d that was solidified by their personal experience of protection in the wilderness.
Sources: Commentary, Devarim 29:3-5