‘For this is the message which you heard from the beginning, that we should love one another, not as Cain was of the evil one, and killed his brother. And why did he kill him? Because his works were evil, and his brother's righteous.'
‘For this is the message which you heard from the beginning.' Compare 1 John 1:5. Here the reference to ‘the beginning' is possibly to the beginning of their Christian lives, for the message is to love one another. Compare 1 John 2:24. Alternatively the thought may be that ‘the beginning' that they heard from was Genesis 2-4 (compare 1 John 3:8). For relationships between brothers have been there since the beginning of mankind. The message that God is light is now being followed by the message that God is holy love, but it is love that is compatible with that light.
John begins the section by concentrating on the importance of Christians loving fellow-Christians, those true to the essentials of the faith. He points out that it has been true from the beginning, even from the time of Adam. For there were two brothers (Genesis 4). One was righteous. He sought to be faithful to the will of God. The other was rebellious. He did not ‘do well'. He did not seek the will of God. And so, instead of repenting, rebellious Cain slew righteous Abel. He revealed himself as of the Evil One. He revealed himself for what he was, a rebel against God, deceived and influenced by Satan (but not a worshipper of Satan. His descendants probably worshipped God as El). And it was revealed in his failure to love his brother.
And why did he not love his brother? Because his own deeds were sinful and his brother's righteous. Thus he became inflamed against his brother and killed him.
In both Jewish and early Christian writings Cain appears as a type of those who refuse to obey God and who refuse to love their brothers. The Testament of Benjamin (John 7:5), for example, looks forward to the punishment of those who “are like Cain in the envy and hatred of brothers.” Philo expanded on the whole theme. Jude speaks of ‘the way of Cain' (Jude 1:11). And so on.