1 John 3:10-12

So now we can tell who are children of God and who are children of the devil. Anyone who does not live righteously and does not love other believers does not belong to God. 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.