Another question which is commonly asked in response to this passage is the following: Is God's grieving in vain? Doesn't He know who will be saved?
The LORD was sorry that He had made man on the earth, and He was grieved in His heart. (Genesis 6:6)All of Scripture is a testimony of God's sovereignty over all of creation. He causes nations to rise and fall. He has all power; the nations are like a drop from a bucket, and are regarded as a speck of dust on the scales. (Isaiah 40:15) God also knew from the beginning who would be saved. In fact, the book of Ephesians states that Christ chose us in Him before the foundation of the world. (Ephesians 1:4)
God's knowledge of who would be saved from the creation of the world does not negate man's responsibility. That God judges men for their sin is evidence of the fact that it is man who chooses to rebel. While God knew before He created the world that some would sin and therefore be subject to eternal punishment, this does not negate the fact that God hates sin. God does not force men to sin. Men choose to sin. And when men sin, it offends God; it grieves Him.
Out of all the men in the pre-flood world, Noah and his immediate family were graciously saved. This was not unjust. As can clearly be seen from the account of the flood in Genesis, the men of the pre-flood world were fully deserving of judgment. God wasn't unjust in choosing only to save Noah and his immediate family; rather, He was acting in unfathomable love!
Throughout history, many have scoffed at the doctrine that God saves who He wills to save. But it is the most merciful doctrine of all. Without God's transforming grace, all men would continue to reject God. God did not spare the angels when they sinned (2 Peter 2:4), but he saves men! That God saves men who have committed such terrible atrocities is an amazing reality to behold!
120 More Years of Mercy - Genesis 6:3
The Total Wickedness of the Pre-Flood World - Genesis 6:5
Righteous Men of the Pre-Flood World - Genesis 4:26
Who Are the Sons of God? (Part 1) - Genesis 6:1-4