What caused God to send a flood upon this earth, resulting in the death of mankind and countless animals? It had to do with the wickedness of man (Genesis 6:5-8). He had forgotten the ways of his Creator. People became overly comfortable with temporal blessings and so had removed the things of God from their minds. They behaved as though He was nonexistent, caring only about the things of this earthly realm (Matthew 24:38; Luke 17:27).
Noah warned his generation of how God would judge the world through a flood. He was the recipient of mocking and scoffing as he prepared an ark for his family. The people were preoccupied with the things of this life: eating, drinking, and marriage. They were not concerned with the things of God. That is why the flood which Noah had spoken of beforehand was unexpected (2 Peter 3:3-6).
Just as God cast judgement on sinful mankind through a flood, so He will judge the unrepentant and unbelieving world again. The imagery of fire is utilized as He refines creation and permanently does away with evil (2 Peter 3:7; 10-13). Both scenarios point us to the beginning of a new creation. Those who are righteous by faith will be delivered from the wrath of God to come (Hebrews 11:7).
Just as faithful Noah was dismissed when he preached repentance, so there are plenty in this fallen world who mock the gospel message proclaimed by God's church. Indeed, He has been rather patient and merciful with all these people (2 Peter 3:8-9). Another point meriting consideration here is that Jesus and the New Testament authors treated the Genesis flood as if it were literal history.