The beginning of the book of Devarim has a recounting of the Sin of the Spies from a slightly different perspective, it makes certain implied ideas more explicit.
דברים א:לב - ובדבר הזה, אינכם מאמינם בה' אלקיכם
Devarim 1:32 - And on this matter, you do not believe in Hashem your G-d.
The basic format of the discussion between Yehoshua & Calev and the 10 tribes is as follows:
- The 10 spies describe how great the Land of Israel is.
- They describe how the people are much bigger and stronger than Am Yisrael.
- Calev ben Yefuneh stresses that we can inherit the land.
- The 10 spies say that we cannot inherit the land, stressing further how insignificant Am Yisrael is in comparison to the nations currently inhabiting the Land of Israel.
- The nation sides with the 10 spies.
- Calev ben Yefuneh and Yehoshua bin Nun stress how good the Land of Israel is, and that our inheriting it is dependent on Hashem being with us, which He is.
- The nation still sides with the 10 spies.
- Divine intervention.
The 10 spies and Calev/Yehoshua argument can be simplified to this:
- The 10 spies say we can't inherit the land because we are weak.
- Calev/Yehoshua say we can inherit the land because Hashem is with us.
Calev/Yehoshua do not address the base claim of the 10 spies. They didn't contradict a single claim of theirs, for that matter. They don't dispute the fact that the fruit brought back was abnormally large, that the cities were well fortified, that huge people inhabited the land.
Their only response was "Their protection has left them, Hashem is with us, do not fear them" (Bamidbar 14:9).
- The 10 spies saw realpolitik
- Calev/Yehoshua saw Divine Providence
Our forefathers sinned in that they didn't believe in the Divine Providence involved in settling Eretz Yisrael. They did not believe that they would be made successful by the Hand of Hashem. So they decided to give up on the mitzvah settling the land.