Jesus forecast many of the signs that will be appearing in the last days and not the least of them is the sign of Noah.
But as the days of Noah were, so also will the coming of the Son of Man be. For as in the days before the flood, they were eating and drinking, marrying and giving in marriage, until the day that Noah entered the ark,and did not know until the flood came and took them all away, so also will the coming of the Son of Man be. – Matthew 24:37-39
This brief statement does not explain what mankind was doing that caused God to take such a drastic action but, it must have been very bad, because we know that God is just and merciful. Something was going on among men that held the danger of irreversible corruption. Indeed we have to look at Genesis to understand what was so bad.
Then the LORD saw that the wickedness of man was great in the earth, and that every intent of the thoughts of his heart was only evil continually And the LORD was sorry that He had made man on the earth, and He was grieved in His heart So the LORD said, “I will destroy man whom I have created from the face of the earth, both man and beast, creeping thing and birds of the air, for I am sorry that I have made them.” But Noah found grace in the eyes of the LORD. – Genesis 6:5-8
The Hebrew word translated wickedness is ra’ and means “bad, disagreeable, malignant”, in other words like a cancer! We know that the prescription today for malignant cancer is to cut it out! If it can caught early enough, it may be cured but it grows fast and soon reaches the point of no-recovery. Was that the problem that brought on the flood? I think so.
Jesus told many stories that reflect the same reasoning, such as the tree that wouldn’t bear fruit (Luke 13:7-9), There was but one answer – grace. Grace is unmerited favor but it is also a given ability to overcome. What we cannot do in our own strength, we can do by grace. That was what Noah received from God. The group Living Waters has produced a film on Noah that highlights the problems of Noah’s day. It is included below and I strongly suggest you watch it before continuing on with this post.
There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown. – Genesis 6:4

There we saw the giants (the descendants of Anak came from the giants); and we were like grasshoppers in our own sight, and so we were in their sight.” – Numbers 13:33