According to Scripture, following the Fall, the people kept getting more and more rebellious before the Lord, until He could take no more of the people’s disobedience.
5 Then the LORD saw that the wickedness of man was great on the earth, and that every intent of the thoughts of his heart was only evil continually.
6 And the LORD was sorry that He had made man on the earth, and He was grieved in His heart.
7 And the LORD said, "I will blot out man whom I have created from the face of the land, from man to animals to creeping things and to birds of the sky; for I am sorry that I have made them."
8 But Noah found favor in the eyes of the LORD.
So God saved only a few people and some of the animals, so that they could repopulate the world in obedience to His will. But immediately after the flood, note what happened:
20 Then Noah built an altar to the LORD, and took of every clean animal and of every clean bird and offered burnt offerings on the altar.
21 And the LORD smelled the soothing aroma; and the LORD said to Himself, "I will never again curse the ground on account of man, for the intent of man's heart is evil from his youth; and I will never again destroy every living thing, as I have done.
22 "While the earth remains, Seedtime and harvest, And cold and heat, And summer and winter, And day and night Shall not cease."
If Noah and his family were truly obedient to the Lord, there would be no need for a sacrifice. Furthermore, this was many years before the sacrifice was ever mentioned in the Law. Noah did this on his own, and not in the will of God.
But the key to our understanding is in the middle of verse 21, where the Lord says, “I will never again curse the ground on account of man, for the intent of man's heart is evil from his youth.”
Even though God seemed to accept their repentant offering, He recognized that the evil nature of humans was still there, and He had to find another way of leading the people wholeheartedly back to Him; and thus began the first all inclusive concession, which is not unlike the way parents deal with their rebellious children, by allowing them to do something against the parents’ will in order to teach them a lesson that will lead to realize that their parents were really correct, so that they would begin to live in their will; or in this case, God's will..