10 In this the children of God and the children of the devil are manifest: Whoever does not practice righteousness is not of God, nor is he who does not love his brother. 11 For this is the message that you heard from the beginning, that we should love one another, 12 not as Cain who was of the wicked one and murdered his brother. And why did he murder him? Because his works were evil and his brother’s righteous.
13 Do not marvel, my brethren, if the world hates you. 14 We know that we have passed from death to life, because we love the brethren. He who does not love his brother abides in death. 15 Whoever hates his brother is a murderer, and you know that no murderer has eternal life abiding in him.
There is a manifest difference between children of God (Christians) and children of the devil (those who have not yet become Christians). The former seek to practice righteousness because of a changed life, and the latter live for self, thus producing sin. Loving one’s fellow Christians shows that a person has passed from death to life. God’s desire for people to love one another can only be realized by those who have turned away from sin and trusted Christ. Otherwise, they have the same murderous heart towards others, even if it shows itself in subtle hatred rather than murder, such as Cain inflicted on Abel. [The Bible Panorama.]