1 John 3:11-12, 23

This is the message you have heard from the beginning: We should love one another. We must not be like Cain, who belonged to the evil one and killed his brother. And why did he kill him? Because Cain had been doing what was evil, and his brother had been doing what was righteous.

And this is his commandment: We must believe in the name of his Son, Jesus Christ, and love one another, just as he commanded us.