The following Dvar Torah was given by Rav Mordechai Kaplan of Yeshivat Lev HaTorah. Hope you enjoy. Have a great Shabbos.
Throughout the Torah there are three major sins that have ramifications until our times. The sin of Adam Harishon is the first "major sin," then we have the sin of the Golden Calf, and we have, in our parsha, the sin of the spies. Due to the "sin" of the spies, Am Yisrael wanders the desert for 40 years, as opposed to directly entering Eretz Yisrael. In addition, Chazal point out that the sin of the spies happened on Tisha B'Av, and "because we cried on that day (tisha B'Av) for no reason, we will cry for the rest of history on that day" (over the destroyed Beit Hamikdash). As we see, the "sin" of the spies ranks very high on the level of the Punishment.
When taking a close look at what the spies did, we have to understand: What exactly was this grave sin that the Meraglim did, and why would it warrant ramifications throughout the history of the Jewish People? The Mergalim merely gave their report, and indeed their words were not false. "The people living in the land are aggressive, and the cities are large and well-fortified" (13:28); this was all true. So what was their sin?
Rav Kalanymos Kalmish Shapira zt"l, hy"d, who was the Rebbe of Piacezna, who continued leading the Jewish People amidst the suffering of the Warsaw Ghetto, explains that the sin was that the Meraglim who were the leaders of the Jewish people, gave their report based on reason and the natural path of the world. They did not speak with faith in Hashem, who can remove all obstacles and go beyond the natural path of the world.
Calev on the other hand, peered beyond the natural, he looked with eyes of great faith at Hashem's infinite power. "We must go forth and occupy the land," (Caleb) said, "We can do it!" Calev did not look at the world of reason and calculations. "Aloh Na'aleh" (13:30), look above reason, beyond nature, because Hashem is with is and there is no road block in the way of Hashem.
In the words of the Aish Kodesh, said in 1940 in the Warsaw Ghetto:
A person needs to say: "Yes, all the logic and facts may be indeed true. The people who inhabit the land may be very strong, and the cities well-fortified, but I still believe in God, who is beyond boundaries, and above all nature. I believe that He will save us. We must go forth and occupy the land. We can do it!" he must say without rationalizing or theorizing, for it is this kind of faith and belief in God that brings salvation closer (Sacred Fire, Worch).
Throughout Jewish history we have overcome the greatest odds. We should all be zocheh, both on an individual level and a national level, to look at the world with total, absolute faith in Hashem.