What does 1 John 4:20 mean?
This verse is the fourth of five times John mentions liars in this letter (1 John 1:10; 2:4, 22; 4:20; 5:10). Love for others, especially fellow Christians, is a primary commandment from Christ. So, a person who exhibits hate for others, but claims to know God, is lying. A liar says he knows God but does not keep His commands (1 John 2:4). A liar denies Jesus is the Christ (1 John 2:22). Here, we are told that a liar says he loves God but hates other people.The second part of the verse adds an explanation, moving from the "seen" to the "unseen." It is more difficult to love someone you cannot see than someone you can see. If a person cannot love those they see, they cannot reasonably claim to love those they cannot see. This verse explicitly declares that a person cannot truly love God while hating other people. The person who claims to love God must also show love for others.
This also ties into the idea that, since we cannot see God in all of His divine essence, love is meant to be the way God is seen. Both in ourselves, and in the world, God's love is meant to be the way humanity "sees" Him.