After expulsion from Eden, Adam and Eve had kids. One of those kids was “Not My People” and his actions caused strife with those who were more like “My People.”
Cain did not read the memo about how God’s people worship God and display His likeness.
God’s People act like they’re made in God’s likeness.
Cain did not. Abel did.
Scripture says, “In the course of time Cain brought some of the fruits of the soil as an offering to the LORD. And Abel also brought an offering– fat portions from some of the firstborn of his flock. The LORD looked with favor on Abel and his offering, but on Cain and his offering He did not look with favor. So Cain was very angry, and his face was downcast.” (Genesis 4:3-5).
It’s not a case of unfairness where God is choosing favorites because Abel won the Chopped Champion competition with a better tasting meal. Nope. God saw that Abel gave God the best he had as an act of worship. Cain “phoned it in” and went through the motions of worship but all the while harboring secret sin of keeping the best for himself. He had a different god in mind.

Questions for reflection:
Why would Cain be angry? Who does he think is God in this relationship? What does that tell you about his heart?
Presumably they had the same upbringing, and they certainly had the same parents. What would make Abel “My People” and Cain “Not My People”? What makes you different from your siblings?
God told Cain in Genesis 4:7, “If you do what is right, will you not be accepted? But if you do not do what is right, sin is crouching at your door; it desires to have you, but you must rule over it.” What characterizes “Not My People”?
How do “Not My People” often react to “My People”? Sin left to fester, will. What happens next?
“Now Cain said to his brother Abel, “Let’s go out to the field.” While they were in the field, Cain attacked his brother Abel and killed him. (Genesis 4:8)
Bring that thought to our modern world with modern Israel. Within the Middle East, how do “Not My People” react to “My People” (whether Christians or faithful Jews)?
How does antisemitism herd all Jewish people into the Not My People camp whether God-fearing or not?