17 And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch. 23 And Lamech said unto his wives, Adah and Zillah, Hear my voice; ye wives of Lamech, hearken unto my speech: for I have slain a man to my wounding, and a young man to my hurt. 25 And Adam knew his wife again; and she bare a son, and called his name Seth: For God, said she, hath appointed me another seed instead of Abel, whom Cain slew. 26 And to Seth, to him also there was born a son; and he called his name Enos: then began men to call upon the name of the Lord.
At this point in Genesis we have a divide among the people. Cain is the father of one line, and Seth of another.
Cain’s line seems to be an industrious and inventive group, they begin building cities, raising herds of cattle, and playing musical instruments. However they also continue the more evil traditions of their father, such as in verse 23 where Lamech admits to murder.
We do not learn about the craft of Seth’s descendants, but we are told that they “began to call upon the name of the Lord.” Many interpret this as meaning that Seth’s line distinguished itself by continuing to worship of God, which practice was no longer universal. It is quite possible that this is what the “sons of God” and “daughters of men” in Genesis 6:2 is referring to. Those that continued to believe in and rely on the Lord were the “children of God” and those that did not were the “children of men.” And for a time these two groups continued separately, but eventually intermingled.
And this idea of two people, one led by God and the other not, is a common theme in the Bible. It occurs most famously between the Jews and the Gentiles, but the first instance of it was with the line of Cain and the line of Seth.