{15} If a man say, I love God, and hateth his brother, he is a liar: {16} for he that loveth not his brother whom he hath seen, how can he love God whom he hath not seen?

(15) As he showed that the love of our neighbour cannot be separate from the love with which God loves us because this last gives rise to the other: so he denies that the other kind of love with which we love God, can be separate from the love of our neighbour: of which it follows, that they who say they worship God, and yet do not regard their neighbours lie shamelessly.

(16) The first reason taken from comparison: why we cannot hate our neighbour and love God, that is, because he that cannot love his brother whom he sees, how can he love God whom he cannot see?

People's New Testament

4:20 If a man say, I love God, and hateth his brother, he is a liar. Yet no one can claim to love God who hates his brother. We cannot love the unseen God when we hate the brother who is in God's image.

Wesley's Notes

4:20 Whom he hath seen - Who is daily presented to his senses, to raise his esteem, and move his kindness or compassion toward him.

Jamieson-Fausset-Brown Bible Commentary

20. loveth not . brother whom he hath seen, how can he love God whom he hath not seen-It is easier for us, influenced as we are here by sense, to direct love towards one within the range of our senses than towards One unseen, appreciable only by faith. "Nature is prior to grace; and we by nature love things seen, before we love things unseen" [Estius]. The eyes are our leaders in love. "Seeing is an incentive to love" [OCUMENIUS]. If we do not love the brethren, the visible representatives of God, how can we love God, the invisible One, whose children they are? The true ideal of man, lost in Adam, is realized in Christ, in whom God is revealed as He is, and man as he ought to be. Thus, by faith in Christ, we learn to love both the true God, and the true man, and so to love the brethren as bearing His image.

hath seen-and continually sees.

Matthew Henry's Concise Commentary

4:14-21 The Father sent the Son, he willed his coming into this world. The apostle attests this. And whosoever shall confess that Jesus is the Son of God, God dwelleth in him, and he in God. This confession includes faith in the heart as the foundation; makes acknowledgment with the mouth to the glory of God and Christ, and profession in the life and conduct, against the flatteries and frowns of the world. There must be a day of universal judgment. Happy those who shall have holy boldness before the Judge at that day; knowing he is their Friend and Advocate! Happy those who have holy boldness in the prospect of that day, who look and wait for it, and for the Judge's appearance! True love to God assures believers of God's love to them. Love teaches us to suffer for him and with him; therefore we may trust that we shall also be glorified with him, 2Ti 2:12. We must distinguish between the fear of God and being afraid of him; the fear of God imports high regard and veneration for God. Obedience and good works, done from the principle of love, are not like the servile toil of one who unwillingly labours from dread of a master's anger. They are like that of a dutiful child, who does services to a beloved father, which benefit his brethren, and are done willingly. It is a sign that our love is far from perfect, when our doubts, fears, and apprehensions of God, are many. Let heaven and earth stand amazed at his love. He sent his word to invite sinners to partake of this great salvation. Let them take the comfort of the happy change wrought in them, while they give him the glory. The love of God in Christ, in the hearts of Christians from the Spirit of adoption, is the great proof of conversion. This must be tried by its effects on their temper, and their conduct to their brethren. If a man professes to love God, and yet indulges anger or revenge, or shows a selfish disposition, he gives his profession the lie. But if it is plain that our natural enmity is changed into affection and gratitude, let us bless the name of our God for this seal and earnest of eternal happiness. Then we differ from the false professors, who pretend to love God, whom they have not seen, yet hate their brethren, whom they have seen.