When asked by a Pharisee which of the commandments is the greatest in God's Law, Jesus replied that it was to love the Lord your God with your whole heart, soul, and mind. He said that this was the first and the greatest commandment (Matthew 22:37-38).
But Jesus didn't stop there. He pointed out that there was a second commandment that was like the first. You must love your neighbor as yourself (Matthew 22:39). The two go hand in hand. We are required to love God first, but we are also required to love our neighbor. All of God's Law is centered on these two commandments (Matthew 22:40).
As God's children, we are called to love all people (1 Thessalonians 3:12). We are to love our neighbor, whether that person is a believer or not. We are even called to love our enemies (Matthew 5:44). We are commanded by God to love our neighbor, to love all people, in the same way that we love ourselves.
Now, we cannot love God while at the same time hating a brother or sister. If we do, we are liars. We can say that we love God, but that would be a lie. How can we possibly love God, who we can't see, yet hate a brother and sister, who we can see? It is not possible. God has commanded that if we love Him, we must also love our brothers and sisters (1 John 4:20-21).
When we love one another--when we love our brothers and sisters in Christ, when we love our neighbors, and even when we love our enemies--then we can truly say that we are born of God and that we know God (1 John 4:7). God is love. When we love one another, God lives in us and His love is made complete in us (1 John 4:12). And the love that we have for one another shows the world that we are true believers--disciples of Jesus Christ (John 13:35).