‘In this the children of God are clearly revealed, and the children of the devil. Whoever does not practise righteousness is not of God, nor he who loves not his brother.'
So the children of God are clearly revealed. They are revealed by the fact that they love God and obey His teachings, that they are those who love their brothers who are true in the faith. This does not make them children of God, it results from the fact that they are. But those who do not seek to obey God's will, who are not ‘righteous', and who do not love those who are true in the faith, simply reveal themselves as what they are, the children of the Devil (that is, those who behave like the Devil in his refusal to listen to God).
It should be noted that the level of righteousness might differ from babyhood to maturity. For the babe in Christ must learn what righteousness is. But the aim of each will now be the same. To please God and be done with sin.
It will be noted that there is no suggestion of being ‘begotten by the Devil'. The Devil works by deceit and treachery, not by begetting. He is an intruder. The contrast with being begotten or born of God is being begotten or born of human flesh. When men are taken by him and ensnared by him it is a work of outrage. It is not the natural order of things. But nevertheless it is their free choice. They too respond because of what they are. They have by free choice taken up their stand against the will of God.
‘Nor he who loves not his brother.' This seeming appendage now introduces a whole section where the main emphasis is on love, both God's love and love for one's brothers and sisters in Christ.