Why did Israel have to wander in the wilderness for 40 years before entering the promised land?
Israel had to wander forty years because of unbelief at Kadesh. They could have entered the promised land immediately, without battle or losing a single young son, but they did not believe God meant what He said. Therefore God, in His wrath, turned them back into the wilderness for that period of wandering.
Les explains that when Israel went up from Sinai to Kadesh, they could have entered the promised land right then. They could have possessed it without losing a single young son and without lifting a sword. But they did not believe that God meant what He said. Because of that unbelief, God in His wrath turned them back into the wilderness for the forty years of wandering. Now, almost forty years later, as they are about to cross the Jordan, they will stop for Passover, and that Passover will be exactly forty years from the Passover in Egypt. Les points out this shows how meticulous God is with His timing. Deuteronomy was written in that final month or two while they sat opposite the Jordan, with Moses reviewing everything since the coming out of Egypt.