19 We love him, because he first loved us.
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.
John starts here letting know that Jesus taught us how to love because He loved us first and showed us by example. Then he goes on to tell us that one who does not love his brother and says he loves God is a liar. If you cannot love someone who you can see how can you love someone who you cannot not see. When we love our brothers on the earth, we are in turn loving Jesus who made them. It is impossible to love God if you do love your brethren. This last verse reminds us of the commandment that Jesus gave when he told the priest that the first commandment was to love God with all your heart and the second was to love your brother.