What does 1 John 3:12 mean?
In contrast with loving one another in verse 11, John brings up the example of the first murderer, Cain. Cain recognized God (Genesis 4), but did not follow Him. Instead, Cain showed that he did not love God by killing his brother. Hatred for one's brother—spiritually speaking—was already condemned as Satanic in 1 John 2:9. There is no sin which a Christian is not somehow capable of, but those who persist in "walking in darkness" cannot reasonably be considered saved.This verse also asks and answers the question of why Cain killed his brother: jealous resentment, since Abel's offerings were more acceptable than Cain's. Jealousy and hatred towards one's brothers and sisters is "of the evil one." Cain's offering was evil in some way, likely because he did not offer his best before the Lord. Abel's offering was considered righteous. John uses this contrast to show the difference between attitudes motivated by God, and those motivated by Satan. Hatred for the good that others do is absolutely evil, and a sign of someone lacking fellowship with Christ.