Albert Barnes Commentary 1 John 4:21

"And this commandment have we from him, that he who loveth God love his brother also." — 1 John 4:21 (ASV)

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 James 2:10.

That he who loveth God love his brother also. See John 13:34 and John 13:35.

Compare John 15:12, 17.