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Albert Barnes and Edward D. Andrews write,
1 John 4:21 Updated American Standard Version (UASV)
21 And this commandment we have from him, that the one who loves God should love his brother also.
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 the one who loves God should love his brother also. This command he gave them as Jesus was about to leave the disciples, to be a badge of discipleship, by which they might be known as his friends and followers, and by which they might be distinguished from all others. It is called new, not because there was no command before which required people to love their fellow man, for one great precept of the law was that they should love their neighbor as themselves Lev. 19:18. Still, it was new because it had never before been made that by which any class or body of people had been known and distinguished. The Jew was known by his external rites, by his uniqueness of dress, etc.; the philosopher by some other mark of distinction; the military man by another, etc. In none of these cases had love for each other been the distinguishing and special badge by which they were known. But in the case of Christians, they were not to be known by distinctions of wealth, learning, or fame; they were not to aspire to earthly honors; they were not to adopt any special style of dress or badge, but they were to be distinguished by tender and constant attachment to each other.
This was to surmount all distinctions of country, color, rank, office, or station in life. Here they were to feel that they were on the same level, had common wants, were redeemed by the same sacred blood, and would receive eternal life by the same Savior. They were to befriend each other in trials; be careful of each other’s feelings and reputation; deny themselves to promote each other’s welfare. See 1 John 3:23; 1 Thess. 4:9; 1 Pet. 1:22; 2 Thess. 1:3; Gal. 6:2; 2 Pet. 1:7. In all these places, the command of Jesus is repeated or referred to, and it shows that the first disciples considered this indeed as the special law of Christ. Moreover, this command or law was new in regard to the extent to which this love was to be carried, for he immediately adds, “As I have loved you, that you also love one another.” His love for them was strong, continued, unremitting, and he was now about to show his love for them in death. John 15:13; “greater love hath no man than this, that a man lay down his life for his friends.” So, in 1 John 3:16, it is said that “we ought also to lay down our lives for our brothers.” This was a new expression of love; it showed the strength of attachment we ought to have for Christians and how ready we should be to endure hardships, encounter dangers, and practice self-denial to benefit those for whom the Son of God laid down his life.—See John 13:34, 35. John 15:12, 17.
Daniel L. Akin,
4:21 Loving one’s brother is not just a spiritual requirement; it also is a command. The reason that it is impossible for the inconsistency stated in 4:20 to remain is that the command to love God and the command to love one’s brother are two parts of one command. They are inseparable. In fact, the use of “and” (kai) to begin the verse connects it with the prior verse. The phrase ap’ autou (“From him,” trans. “he has …”) is a reference to God the Father as the final source of the commandment. Disobedience to this commandment demonstrates a false love toward God that results in failure to love the brethren. As Paul wrote: “The entire law is summed up in a single command: ‘Love your neighbor as yourself’ ” (Gal 5:14). Of course, Paul and John therefore agree with their Lord (Mark 12:31).
 Albert Barnes, Notes on the New Testament: James to Jude, ed. Robert Frew (London: Blackie & Son, 1884–1885), 335.
 Daniel L. Akin, 1, 2, 3 John, vol. 38, The New American Commentary (Nashville: Broadman & Holman Publishers, 2001), 187–188.