“And this commandment have we from him, that he who loveth God love his brother also.”
This verse almost certainly relates to the great summary of all the law and the prophets as given by Jesus Christ in these words:
The first (and great commandment) is, Hear, O Israel; the Lord our God, the Lord is one; and thou shalt love the Lord thy God with all thy heart, and with all they soul, and with all thy mind, and with all thy strength. The second is this, Thou shalt love thy neighbor as thyself. There is none other commandment greater than these (Mark 12:29-31).
That almighty God desires that his human creation should love him is one of the most revealing statements in Scripture. That purposeful desire of God lies back of all that God ever revealed, all that he ever did, to redeem mankind. How universally do people tend to fall short of this basic love! Much of the love that passes for such is merely bigotry. “The bigot loves those who embrace his opinions,” and receive his peculiar bias or prejudice; and he loves them for that, not for Christ.
Concerning Jesus’ joining in this verse and in the Gospels these twin commandments to love God and love one’s neighbor, Stott remarked, “What Christ has joined, let no man sever.” Plummer’s summation of John’s thoughts here is as follows:
Here 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 the Divine command; let us remember the Divinity which even the most debased humanity contains.
May our attitude toward this holy commandment be that of freely confessing that the total fulfillment of it lies utterly beyond our unaided human strength to accomplish it; but may we also preempt unto that holy purpose the blessed promise of the apostle: “I can do all things in him that strengtheneth me” (Philippians 4:13). “In him?” Yes, “in Christ,” in whom we shall at last be presented before the Father in perfection!
What a wonderful world this would be, and what an incredible sweetness would pervade it, if even any appreciable percentage of its population would live by the principles laid down in this chapter of the word of God!
 John Wesley, op. cit., p. 916.
 John R. W. Stott, op. cit., p. 171.
 A. Plummer, op. cit., p. 105.