11 “He that says he is in the light”— now he is making all clear that he has been saying— “he that says he is in the light, and hates his brother, is in darkness even until now.” What! My brethren, how long shall we say to you, “Love your enemies”? See whether, what is worse, you do not hate your brethren. If you loved only your brethren, you would be not yet perfect: but if you hate your brethren, what are you, where are you? Let each look to his own heart: let him not keep hatred against his brother for any hard word; on account of earthly contention let him not become earth.
For whoso hates his brother, let him not say that he walks in the light. “He that says he is in the light, and hates his brother, is in darkness even until now.” Thus, some man who was a pagan has become a Christian; mark well: behold he was in darkness, while he was a pagan: now is he made henceforth a Christian; thanks be to God, say all joyfully; the apostle is read, where he says joyfully, “For you were sometime darkness, but now light in the Lord.” Once he worshipped idols, now he worships God; once he worshipped the things he made, now he worships Him that made him.
He is changed: thanks be to God, say all Christians with joyful greeting. Why? Because henceforth he is one that adores the Father and the Son and the Holy Ghost; one that detests demons and idols. Yet still is John solicitous about our convert: while many greet him with joy, by him he is still looked upon with apprehension. Brethren, let us gladly welcome a mother's solicitude. Not without cause is the mother solicitous about us when others rejoice: by the mother, I mean charity: for she dwelt in the heart of John, when he spoke these words.
Wherefore, but because there is something he fears in us, even when men now hail us with joy? What is it that he fears? “He that says he is in the light”— What is this? He that says now he is a Christian—“and hates his brother, is in darkness even until now.” Which there is no need to expound: but to be glad of it, if it be not so, or to bewail it, if it be.
Source: Homilies on the First Epistle of John (New Advent)