John Gill Commentary 1 John 4:20

"If a man say, I love God, and hateth his brother, he is a liar: for he that loveth not his brother whom he hath seen, cannot love God whom he hath not seen." — 1 John 4:20 (ASV)

If a man say I love God, and hates his brother
Nothing can be more contradictory than this profession, not black and white, or hot and cold in the same degree:

he is a liar ;
it is not truth he speaks, it is a contradiction, and a thing impossible:

for he that loves not his brother whom he has seen ;
his person, which might have drawn out his affection to him; and something valuable and worthy in him, which might have commanded respect; or his wants and distresses, which should have moved his pity and compassion:

how can he love God whom he has not seen ?
it cannot be thought he should; the thing is not reasonable to suppose; it is not possible he should; (See Gill on 1 John 4:12).