All of you approached me and said, "Let us send men ahead of us and let them spy out the land". (Deut. 1:22) Moshe Rabenu, his words of rebuke to the nation by reminding them of their sins and rebellions. One of the events he mentioned was the incident of the spies. The Jews believed their slanderous report concerning the land of Israel, and did not believe Yehoshua and Calev who reported that the land was in fact a very good place.
It is difficult to understand this because Yehoshua and Calev were outnumbered ten to two. Why were the Jewish People reprimanded for not listening to Yehoshua and Calev - mustn't one follow the majority?
Although Yehoshua and Calev were outnumbered, logic would suggest that their report had a much stronger basis than the report from the other 10. The 10 spies said that they should not enter the land, but no one would ever know if they were telling the truth. However, Yehoshua and Calev said that the people should enter the land, and they knew that when the Jews would enter they would see for themselves that Yehoshua and Calev had been telling the truth.
The sin of believing the slanderous report of the 10 spies was that the people did not use their intellect. They should have taken a step back and pondered the situation, and then they would have realized the real truth.
The gravest sin a person can be guilty of is to abandon the gift of intellect that Hashem gave each of us. G-d instilled every human being with intelligence and understanding with which he is capable of grasping divine wisdom. We must think before we do, say or behave in certain ways. Are we doing it just because everyone is doing it? Or do we stop and think: Is it the right thing?
It is our intellect that puts us above the animal kingdom and gives us the ability to reach tremendous heights.