1 John 4:20-21
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, how can he love God whom he hath not seen?
21 And this commandment have we from him, That he who loveth God love his brother also.
The Grace New Testament Commentary - 1 John 4:20
4:20 To a Christian it might seem easier to love God whom he has not seen than to love his brother whom he has seen. While God seems perfectly deserving of the Christian's love, one's brother often does not. But for John, love is not an emotional word. For John "to love" is to behave in a way that meets the need of one's fellow Christian (see 1 John 3:16-18). By their actions, not their feelings, John assesses the reality of the believers' love for one another: they are to love "in deed" and therefore "in truth" (1 John 3:18).
Since action, not emotion, is the critical issue in Christian love, it is obvious that there is actually no basic difference between expressing love for God and love for a Christian brother, since the test of one's love for Him is obedience to His commandments (see 1 John 4:19). If a Christian does not obey God's commandments, he does not love Him, no matter what he says or feels. Thus the person who says, "I love God," but does not obey God's command to love his brother, is a liar.