We love because He first loved us. If anyone says, “I love God,” but 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 we have this commandment from Him: Whoever loves God must love his brother as well. – 1 John 4:19-21
Loving God, and loving others. You cannot have one without the other.
We love God because He has shown us His own great love. He has commanded us to love one another, and has set the perfect example through His son, Jesus. If we do not love others, then we are disobeying His command, and we certainly are not acting like His children.
We demonstrate that God has changed us and that His Spirit is working in us by loving others. It will be apparent that we love others because our actions and motivations will demonstrate it. We can see others, and they can see us and how we behave. God may not be visible in the same way, but through our love we show Him clearly.
Again: Loving God, and loving others. You cannot have one without the other.