It is customary for the Bible to list the firstborn first, but not mandatory. Genesis 4:1 gives the first time a child of Adam and Eve's is mentioned, but it does not necessarily follow that Cain was their firstborn. There are several unprovable presumptions in this line of thinking: So of whom was Cain afraid? Does this prove that God made another race of people before Adam and Eve (because the text doesn't mention any other people, except Cain's wife in verse 17)? God curses Cain with a difficult life and exile, and in verse 14, Cain expresses fear that someone else will be aware of his vulnerable position and kill him. God accepts Abel's sacrifice, but not Cain's (we're not told why, but it's speculated that it's either because Cain didn't sacrifice with a pure heart or only Abel's required the shedding of blood). In the ensuing verses, Cain sacrifices crops to God while Abel sacrifices sheep. Genesis 4 gives the account of Cain and Abel. After Cain killed Abel, whom did he have to fear?

Cain and abel bible verse