The Catechism in Questions & Answers

226. How was the further development of mankind after the fall into sin?

After the fall into sin, the sins of mankind increased at an appalling rate: first Cain slew his brother Abel even despite God’s warning (cf. Genesis 4: 6-8).
As time progressed, human beings began to sin more and more. God decided to punish them and sent the great flood. Only Noah found grace in the eyes of God. At God’s commandment Noah built an ark in which he and his family were saved (cf. Genesis 6: 5-7, 17-18).
Even after this judgement, human beings persisted in their disobedience toward God. For example, the Bible tells of the tower of Babel. God caused the builders of the tower to fail in their endeavour owing to their arrogance and striving for fame: He confused their languages so that they could no longer understand one another (cf. Genesis 11: 1-8).

“Now Cain talked with Abel his brother; and it came to pass, when they were in the field, that Cain rose up against Abel and killed him.”
Genesis 4: 8