Welcome to answers from an apostolic faith.
In name of the Father and Son, the Holy Spirit, One God. Amen.
How can a loving God flood the earth? How can He destroy the humanity He once created? On a superficial level, this question seems very perplexing, but let’s dig a bit deeper. First, we need to understand the level of sin humanity was in at the time. If we go back to when Cain killed his brother Abel, God told him: A fugitive and a vagabond you shall be on the earth. So, Cain’s reply was: It will happen that anyone who finds me will kill me. He was afraid of being killed. This response implies that knowledge of killing was familiar to humanity.
Already, in the second generation, corruption had deeply affected humanity. Then, we get a hint of the level of the third generation when Seth, the third son of Adam and Eve, begets Enosh. When Enosh was born, the Bible says: Then men began to call on the name of the Lord. This sentence implies that everyone else, or almost, had completely abandoned God. We're not talking about people struggling with sin, we are talking about people that willingly deserted God. Undeniably, it was now a matter of time before sheer disaster. And then we see the verse that clarifies it all. It says:
'' 5 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.'' (Genesis 6:5)
Imagine a place where every intent of the heart of all the humans is continually evil. I don't think we can begin to imagine it. Think about it. Although there's so much evil in today's world, we see murders, wars, rapes, corruption, violence, you name it, but in spite of this, there are still plenty of people doing God’s work every day. There are lots of people that live the commandments of God and are being the light of the world. Today’s humanity has not attained the level of corruption that was prevalent at the time of Noah, far from it. Understanding this is the beginning of the answer to this puzzling question.
The God Christians worship is a sovereign God that is perfectly just in His mercy and perfectly merciful in His justice. The same way a human will disagree with a killer being freed when he's evidently guilty, it is not a surprise that God will put an end to the propagation of evil. But did God rush to annihilate humanity? Let's see. In Genesis 6:3, says the following:
'' 3 And the Lord said, “My Spirit shall not strive with man forever, for he is indeed flesh; yet his days shall be one hundred and twenty years.'' (Genesis 6:3)
The average reader thinks that this verse means that humans will not live beyond the age of 120. However, this is not what the verse is saying. It is saying that humanity as a whole has 120 years to repent from their evil. That’s a very long time. And as St. Peter declares, Noah was preaching repentance to humanity. However, the response to this preaching was not very positive. Christ shows this when He says that they focused on earthly things like eating and drinking. In addition to this, in Genesis 7, verse 10, after Noah and his family went inside the ark, we see that God gave humanity an additional 7 days, an additional chance for repentance. But no one repented.
But could God have waited 130 years instead of 120? Or 150? Would it have changed anything? We find the most significant rationale for the flood when we look at the holistic story of salvation. We must remember that the Holy Trinity willed to send the Son of God to take flesh and become the God-man we worship today. And the humanity of Christ is absolutely vital to our salvation. As St. Gregory the Theologian says:
''For that which He has not assumed, He has not healed; but that which is united to His Godhead is also saved.'' (St. Gregory the Theologian)
In other words, God had to take on our full humanity to be able to save us. This humanity of Christ needs to come from a human, namely St. Mary. Think about it from this perspective: during the time of Noah, all of humanity was corrupt and there was only one family left. This is the family that eventually brought forth Abraham, Isaac and Jacob who became the people of God in the Old Testament, and most importantly, brought forth St. Mary, the bearer of God. God took flesh from her to save the entire human race, those before and after the contemporaries of Noah.
What would have happened if this family also had become corrupt? Thankfully, we do not need to answer this question because our sovereign God is in control. Looking at it from this perspective, we once again find that God patiently endured the sins of humanity and waited till humanity gave Him no other choice but to proceed with the flood. He had to do it for the salvation of all those who are seeking Him from Adam to those of the last generation. The flood occurred not only because God is just, but also because God loves us and wants to save us. Although what jumps at us in this story is the justice of God, if we look a bit deeper, we find also His patience, His mercy, His love.
Remember: Know your faith, live your faith, and teach your faith. And glory be to God forever. Amen.