The decree of the Jews being 40 years in the desert alludes to the effect and purpose for their stay there. The number 40 is associated with the ability to understand. The incident of the spies demonstrated that the Jewish people didn’t fully understand the message of the Torah. These 40 years brought them to to a higher level of Bina (understanding). When the 40 years passed they were able to enter the earthiness of the land and accomplish their mission there.
