1 John 3:12
 We should not be like Cain, who was of the evil one and murdered his brother. And why did he murder him? Because his own deeds were evil and his brother's righteous. (ESV)
The Apostle John is talking to the church. He has been speaking of how to identify those who are genuinely God's children and those who are children of the devil. Now, he turns to this example from the very first human family to talk of how not to act. At first glance it may seem a little odd to be speaking of Cain. Let's take a look at why John warns the church of Cain's sinful example.
 In the course of time Cain brought to the LORD an offering of the fruit of the ground,  and Abel also brought of the firstborn of his flock and of their fat portions. And the LORD had regard for Abel and his offering,  but for Cain and his offering he had no regard. So Cain was very angry, and his face fell. (ESV)
Cain gives some kind of evidence that he was a worshiper. He was bringing an offering to the Lord. Yet, his heart was clearly not right before God. Notice that Cain brings "an offering." Abel brings the "firstborn," and "fat portions." To help you understand, Abel brought the very best to the Lord, and what the Lord wanted. Cain brought what he determined was right to bring. Cain's heart was rebellious. And when he did not get what he wanted he exposed his true heart; jealous, angry, and murderous.
Do you know who else was like Cain? The religious leaders of the Jews who had Jesus crucified. Their deeds were evil and Jesus' deeds were righteous. Consumed by their jealous rage they were willing to abandon the Law they claimed to believe and resort to murder. It is a terrifying thing to think of how quickly jealousy can turn to even greater sin.
The false teachers in the Church that John was addressing had a superior attitude towards those who were practicing true righteousness. Their hearts were evil, just like Cain's. John rightly instructs them to not be like Cain. They also needed to be aware of what was really happening in their midst. I cannot stress strongly enough how important it is for churches to discern the hearts of its members.
How is your heart? Are you jealous of others that seem to be more righteous than you? Instead, humble your heart before the Lord. As God told Cain in Genesis 4:6, "If you do well, will you not be accepted?" Lord help us to do well.