The punishment that God brought upon Miriam was meant as a lesson of the severity with which God punishes slander. For Miriam spoke no evil of Moses in the presence of any one except her brother Aaron. She had moreover no evil motive, but a kindly intention, wishing only to induce Moses to resume his conjugal life. She did not even dare to rebuke Moses to his face, and still, even in spite of her great piety, Miriam was not spared this heavy punishment.  Her experience, nevertheless, did not awe the wicked man who, shortly after this incident, made an evil report of the promised land, and by their wicked tongues stirred up the whole people in rebellion against God, so that they desired rather to return to Egypt than to enter Palestine. The punishment that God inflicted upon the spies as well as upon the people they had seduced was well deserved, for had they not been warned of slander by Miriam's example, there might still have been some excuse. In that case they might have been ignorant of the gravity of the sin of slander, but now they had no excuse to offer. 
When Israel approached the boundaries of Palestine, they appeared before Moses, saying: "We will send men before us, and they shall search out the land, and bring us word again by what way we must go up, and into what cities we shall come." This desire caused God to exclaim: "What! When you went through a land of deserts and of pits, you had no desire for scouts, but now that you are about to enter a land full of good things, now you wish to send out scouts. Not only was the desire in itself unseemly, but also the way in which they presented their request to Moses; for instead of approaching as they had been accustomed, letting the older men be the spokesmen of the younger they appeared on this occasion without guidance or order, the young crowding out the old, and these pushing away their leaders.  Their bad conscience after making this request - for they knew that their true motive was lack of faith in God - caused them to invent all sorts of pretexts for their plans. They said to Moses: "So long as we are in the wilderness, the clouds act as scouts for us, for they move before us and show us the way, but as these will not proceed with us into the promised land, we want men to search out the land for us." Another plea that they urged for their desire was this. They said: "The Canaanites fear an attack from us and therefore hid their treasures. This is the reason why we want to sent spies there in time, to discover for us where they are hiding their treasures." They sought in other ways to give Moses the impression that their one wish was exactly to carry out the law. They said: "Hast not thou taught us that an idol to which homage is no longer paid may be used, but otherwise it must be destroyed? If we now enter Palestine and find idols, we shall not know which of them were adored by the Canaanites and must be destroyed, and which of them were no longer adored, so that we might use them." Finally they said the following to Moses: "Thou, our teacher, hast taught us that God 'would little by little drive the Canaanites before us.' If this be so, we must send out spies to find out which cities we must attack first."  Moses allowed himself to be influenced by their talk, and he also liked the idea of sending out spies, but not wishing to act arbitrarily he submitted to God the desire of the people. God answered: "It is not the first time that they disbelieve My promises. Even in Egypt they ridiculed Me, it is now become a habit with them, and I know what their motive in sending spies is. If thou wishest to send spies do so, but do not pretend that I have ordered thee."
Moses hereupon chose one man from every tribe with the exception of Levi, and sent these men to spy out the land. These twelve men were the most distinguished and most pious of their respective tribes, so that even God gave His assent to the choice of every man among them.  But hardly had these men been appointed to their office when they made the wicked resolve to bring up an evil report of the land, and dissuade the people from moving to Palestine. Their motive was a purely personal one, for they thought to themselves that they would retain their offices at the head of the tribes so long as they remained in the wilderness, but would be deprived of them when they entered Palestine.