Albert Barnes' Notes on the Whole Bible - 1 John 4:21
And this commandment have we from him - That is, the command to love a brother is as obligatory as that to love God. If one is obeyed, the other ought to be also; if a man feels that one is binding on him, he should feel that the other is also; and he can never have evidence that he is a true Christian, unless he manifests love to his brethren as well as love to God. See the notes at James 2:10.That he who loveth God love his brother also - See the notes at John 13:34-35. Compare John 15:12,... read more
The Pulpit Commentary - 1 John 4:21
That he who loveth God love his brother also. This is the great commandment, on which hang all the Law and the prophets ( Matthew 22:37 , Matthew 22:39 ; Luke 10:27 ; John 13:34 ), and, whatever we may think of the relation between seeing and loving, there is the Divine command to love, not only the invisible God, but the visible brother in whom the invisible God dwells. Sight may hinder as well as help; it is hard to love what is squalid and hideous. In such cases let us remember... read more