30 But Caleb quieted the people before Moses and said, "Let us go up at once and occupy it, for we are well able to overcome it."
31 Then the men who had gone up with him said, "We are not able to go up against the people, for they are stronger than we are."
These verses show the unbelief of the people which condemns them to wander the desert for 40 years.
Their unbelief is contrasted against the faith of Caleb who believed they could do it because God would be with them to achieve all that he had promised them.
It is remarkable that Israel continued to fail to believe in God's ability to deliver them and to bless them. It is as though they didn't even believe that he existed despite all that he had done in signs, wonders, blessings, and delivering them from Egypt. We could imagine that they must have been pretty dense to fail when all of this was right before them, but they were not unintelligent - they merely did not believe. Their hearts were unchanged from their fallen nature and their unbelief reflected their internal wickedness.