How do we know we have the real thing? Remember, that is one of the key questions John is answering in this letter - How do I know if I have the real thing? How do we know we are reflecting Jesus and living as children of God? The answer is LOVE.
John is saying, "If God is love, and if God lives in us, then love should be evidenced in our lives." ...and John doesn't leave this alone. He raises the stakes...
"Anyone who hates a brother or sister is a murderer, and you know that no murderer has eternal life residing in him." 1 John 3:15
Whoa - slow down there John... aren't you going a little overboard. Come on...comparing hate to murder...come on... or is there something to this?