Let's back up to v16b where it says God is love. Whoever lives in love lives in God, and God in him. V19 We love because He first loved us. V20 If anyone says, "I love God yet hates his brother, he is a liar. For anyone who does not love his brother, whom he has seen, cannot love God, whom he has not seen. And He has given us this command: Whoever loves God MUST (my emphasis) also love his brother
Frankly, I think no interpretation is necessary. God makes it very plain what He expects from us.
The confidence we have in knowing that God loves us delivers us from fear but not from responsible action. In fact, God's love for us and in us sets us free to love our brother and sister even as God loves them. To fail this test of love proves that one's claim to love God is a lie. John makes clear that obedience expresses itself in a single command. Love for God and love for neighbor are inseparable. The one is not possible apart from the other. Those who love God cannot refuse love to the image of God that meets them in their fellow believers.
Christians should love one another because God is love; we love Him because he loved us first; if a Christian claims to love God but hates his brother, he is a liar. These are strong words. Jesus Christ applied the words “manslayer” and “liar” to Satan the Devil. (John 8:44) Never would we want those terms applied to us!
The apostle John was writing a letter to a christian congregation in the first century, so he was telling the christians that if they don't love their fellow christians they don't love God.
