Verse 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 Barnes "Jas 2:10".

That he who loveth God love his brother also. See Barnes "Joh 13:34, See Barnes "Joh 13:35".

Comp. Joh 15:12,17.

{e} "That he who loveth" Joh 13:34