John writes these interesting lines:
We must not be like Cain who was from the evil one and murdered his brother. And why did he murder him? Because his own deeds were evil and his brother's righteous. Do not be astonished, brothers and sisters, that the world hates you (1 John 3.12-13).
Cain's murder of Abel, according to John, took place out of a certain spiritual jealousy. Seeing his brother was a reminder of Cain's own rejection by God. Abel's sacrifice was accepted, but Cain's was not, and the presence of Abel in the world served nothing else for Cain except to remind him of this unfortunate fact. Out of a kind of spiritual jealousy, mixed perhaps with self-loathing and wounded pride, this hatred for his own brother turns Cain into the first murderer and fratricide.
But what is the connection with the church to whom John is writing? What's his point? It's this: in the same way, people in the world, when they see that you are different from them, that you are better than them, will hate you for it. And this hate, if they don't put any limits to it or deal with it in a healthier way, will turn into murder before long -- as Christians would experience.
This Cainish hate is born of spiritual jealousy. It thinks like this: Who does he think he is? He thinks he's so good, thinks he's better than me. Where does he get off? It is miserably oblivious, or perhaps to put it a different way, it is self-aware in a defective manner. Cain is reminded by Abel that he did wrong and that his sacrifice was not accepted by God. But rather than admit fault and repent, rather than taking responsibility for his own life, he determines the better course of action is to kill his brother, to erase forever from the face of the earth that reminder of Cain's own inadequacy.
Sin is deceptive and irrationally self-destructive in this way. Not only does Cain feel bad about being rejected, but now he carries the burden of having murdered his own brother. This is why Jesus paid so much attention to our emotions and our inner life in the sermon on the mount. Lust, if you do not keep watch over it, will translate into adultery, and you'll be convinced at that time that adultery is the best course of action for you to take. Hatred and anger, too, if you do not keep watch, will translate into murder, and in the same way, you'll be convinced that you're doing the right thing.
John repeats this lesson when he says: All who hate a brother or sister are murderers, and you know that murderers do not have eternal life abiding in them (v. 15). The person who harbors hate in his heart is setting herself up for damnation -- not because Christ wills to damn that person, but because she is orienting herself for willed exclusion from the kingdom of God. I agree with Joseph Ratzinger when he says: Christ inflicts pure perdition on no one. In himself he is sheer salvation (Eschatology, p. 205). Rather what happens is that a person who lives her life in persistent sin, through her own fault, becomes ill-disposed to live in the fellowship of God's kingdom. Certainly a person full that of spiritual jealousy that says, They're not so good!, cannot stand the company of the righteous!
So we have to keep a watch on ourselves, and beware of what sort of emotions are at work in us.