Moses and Aaron were not allowed to go into the Promised Land because of their sins described in Numbers 20. The sins they committed were grievous in God’s sight. They lost faith in God and they did not follow His instructions to them; instead, they thought that they had a better plan. So, because of their sins, they were kept from receiving the joy of entering the Promised Land. In Deuteronomy, Moses was reminded of his sin as he faced the end of his life.
This story presents a perfect picture of why mankind needs a Savior. If we had to earn entrance to Heaven on our own merits and because of our own righteous living, most of us would be turned from the Promised Land Heaven just like Moses was. Because of the blood of Christ, however, we enter Heaven on the merit of Jesus. “And the LORD spake unto Moses, saying, Take the rod, and gather thou the assembly together, thou, and Aaron thy brother, and speak ye unto the rock before their eyes; and it shall give forth his water, and thou shalt bring forth to them water out of the rock: so thou shalt give the congregation and their beasts drink. And Moses took the rod from before the LORD, as he commanded him. And Moses and Aaron gathered the congregation together before the rock, and he said unto them, Hear now, ye rebels; must we fetch you water out of this rock? And Moses lifted up his hand, and with his rod he smote the rock twice: and the water came out abundantly, and the congregation drank, and their beasts also. And the LORD spake unto Moses and Aaron, Because ye believed me not, to sanctify me in the eyes of the children of Israel, therefore ye shall not bring this congregation into the land which I have given them. This is the water of Meribah; because the children of Israel strove with the LORD, and he was sanctified in them” (Num. 20:7-13).