One of the great questions coming out of a reading of Deuteronomy may be asking why God did not let Moses go into the Promised Land? After all, Moses was the great friend of God's, the holiest man alive, and a great leader of God's People.
OK, so he struck the rock more than once...but was the sin merely impatience, or anger? Moses was instructed to speak to the rock and, instead, he hit it twice.
Some commentators say that Moses made himself equal to God by referring to "We" providing the water.
Some commentators state that it was because Moses was disobedient and did not follow God's directions exactly, which Moses obviously did not do.
I think it was impatience. Why is impatience such a serious sin? Impatience is not trusting in God.
After all those years, of intimacy with God, something in Moses still did not trust God enough to wait, wait on the words spoken to the rock for water.
Anger is not trusting in God. Anger is playing God-"I want this, now!" A lack of trust is a lack of love. At that moment, Moses simply did not love God enough to trust in Him.
But, not to go into the Holy Land was a serious punishment.
Moses sinned in front of the entire people. His sin was not private, but public. He gave a bad example of one's relationship to God to the entire assembly of the tribes of Israel.
Moses was not perfect and this surprises us, as only the perfect see God. But, that is the analogy here. Moses, the Giver of the Law, did not enter the place of promise as he was not perfect.
Joshua and Caleb had pure hearts, as did those who were the children of the slaves of Egypt.
A new standard was necessary to enter, and conquer the Promised Land. That standard was purity of heart.
Even though Moses is one of the greatest men of the Old Testament, and represents the Law in the Transfiguration, as Elijah represents the Prophets, and Christ fulfilling the Law and the Prophets, still Moses sinned and was punished. His punishment, however, allowed him to be purified before death. How to we know that? Elijah did not die, but went up to heaven in the fiery chariot. Moses is said in all the commentaries to have died, but his state of perfection was such to allow him to be in heaven, so we see him in the glory of Christ in the Transfiguration. If Moses had been in Hades, in that upper chamber called the First Limbo, waiting for the Harrowing of Hell, he would not have appeared with Christ.
Only the pure enter heaven, and Moses' terrible punishment of not entering Canaan, made his heart pure.
May I add that in the Epistle of Jude, verse 9 notes that the Archangel Michael fought the devil over the body of Moses. This is a great line upon which to meditate and gives teeth to the idea that Moses, like Elijah, was in heaven body and soul.
9 But when the archangel Michael, contending with the devil, disputed about the body of Moses, he did not presume to pronounce a reviling judgment upon him, but said, “The Lord rebuke you."