1 John 3:11-12, 23

This is the teaching you have heard from the beginning: We must love each other. Do not be like Cain who belonged to the Evil One and killed his brother. And why did he kill him? Because the things Cain did were evil, and the things his brother did were good.

This is what God commands: that we believe in his Son, Jesus Christ, and that we love each other, just as he commanded.