Hebrews 3:19

And we see that they were not able to enter in because of unbelief.

And we see is a transitional phrase which means, "We see in the familiar record of the Pentateuch," or "We see in the details just mentioned." This passage shows that the exclusion of Israel grew out of moral necessity, their unbelief having betrayed them into outright rebellion against God. The application, of course, is that, if God spared not them, neither will he spare Christians guilty of the same conduct.

That lost generation of the Israelites suffered incredible hardships in the wilderness, being subject to the incursions of armed enemies, enduring hunger and thirst and wretchedness, being exposed to the sickening agonies inflicted by poisonous serpents, finding no certain habitation, marching every day of their lives in step with frustration, disease and death. And yet it all could have been different. God gave them the right to enter Canaan immediately upon their coming out of Egypt, but through unbelief and disobedience they failed to enter, Never, perhaps, in human history is there so clearly outlined a case in which the religious and spiritual failures of a people issued so promptly and irrevocably in their temporal and physical poverty as well, leaving the lesson for all to see. Moffatt commented on this in these words,

The world at large may ridicule the idea that a man's spiritual standing can have the remotest connection with the success or failure which may attend his pursuit of temporal objects: and we are far enough from alleging that the maintenance of religious principle will necessarily insure the prosperous issue of every enterprise; but its absence may, at any time, throw obstacles in the way which might not, under other circumstances, require to be encountered; and when we find that unbelief and nothing else was the cause of the exclusion of so many Israelite wanderers from the choice and productive land of Canaan, we seem to read, in characters so plain that only willful error can mistake their meaning, the great truth that the earthly prospects of all may be materially and even vitally affected by the possession or the want of faith.[15]