As this week's Torah portion of Shelach

In general, the Jewish people's entry into the Land of Israel is symbolic of the Jew's raison d'etre and indeed, his very function in the world. The Jewish soul descends to the physical plane for the purpose of imbuing it with holiness, transforming the material world into an appropriate vessel for G-dliness. Similarly, the objective of the Jews' entry into the Land was to transform it from "the land of Canaan" into "the Land of Israel," a place where holiness and G-dliness would be openly perceptible.As this week's Torah portion of Shelach relates, before the Jews entered the Land, G-d commanded Moses to "Send men, that they may spy the Land of Canaan." Whenever a Jew is about to perform a commandment, the first step must be to carefully consider the task at hand and find the best way to achieve the objective. The Spies were sent to determine the most effective military strategy to conquer Canaan, within the confines of the natural order.
A Jew might think that once G-d has commanded him to perform a mitzva (commandment), he can ignore reality and close his eyes to his surroundings. However, the story of the Twelve Spies teaches that faith in G-d is not enough. A Jew is required to "use his head," to utilize his G-d-given intellect and abilities to determine the very best way to fulfill His wishes. For G-d has created a physical world, with the intention that mitzvot be performed within the natural order.
At the same time, one mustn't go too far in the opposite direction. The Spies' mistake was that they interpreted their fact-finding mission as permission to decide whether the Jews should enter the Land of Israel at all. This, in essence, was their sin: Moses sent them to determine how to achieve their goal, yet they assumed the right to determine if the Jews should do it in the first place. This led them to their conclusion that "We are not able to go up against the people; for they are stronger than us."
This was contrary to G-d's will, and demonstrated a serious lack of faith. For whenever G-d sends an individual on a mission, He simultaneously gives him the power to succeed. G-d demands of a person only "according to his abilities." If it is illogical for a human being to require another to perform an act beyond his capabilities, how much more so does this apply to G-d, the Essence of goodness and kindness.
With this firm foundation in mind, the Torah goes on to caution that "One mustn't rely on miracles." A Jew is obligated to work within the natural order, not above it. Nonetheless, we are assured of Divine assistance whenever we encounter obstacles, so that we too may declare: "Let us go up at once, and possess it; for we are well able to overcome it."

Adapted from Volume 13 of Likutei Sichot