1 John 4:21

And this commandment have we The Apostle drives home his arguments for the practice of brotherly love by the fact that God has commanded all who love Him to love their brethren. Some take -Him" to mean Christ. But this is unlikely, as Christ has not been mentioned for several verses: although it must be admitted that S. John is so full of the truth that -I and My Father are one", that he makes the transition from the Father to the Son and from the Son to the Father almost unconsciously. Where has God given this commandment? In the whole Law, which is summed up in loving God with all one's heart and one's neighbour as oneself (Deuteronomy 6:5; Leviticus 19:18; Luke 10:27). The Apostle thus anticipates a possible objection. A man may say -I canlove God without loving my brother, and I can prove my love by keeping His commandments" (John 14:15). -Nay", says S. John, -your own argument shews your error: you cannot keep His commandments without loving your brother". Thus then we have two revelations of God: our brother, who is His image; and His commandment, which is His will. Not to love our brother is a flagrant violation of both. As Pascal puts it, we must know men in order to love them, but we must love God in order to know Him.

that he who loveth God love his brother also "The final particle (ἵνα) gives more than the simple contents of the commandment. It marks the injunction as directed to an aim" (Westcott). See on 1 John 1:9.