Love Your Neighbor
"If anyone says, 'I love God,' and hates his brother, he is a liar; for he who does not love his brother whom he has seen cannot love God whom he has not seen."
John says if we do not love one another, that's evidence that we don't really love God. It's easier to say "I love God" than to actually love our neighbors. Only if we have love for God will we truly be able to love our neighbors, therefore obeying the command to love our neighbors is a demonstration of our love for God.
This parallels what James says about faith. It's easy to say you have faith, but when faith is present there will be external evidence (works). Paul is correct that we are saved by faith alone not faith plus works. However, that faith will produce works. So also love for God will always display itself as love (an admittedly imperfect love in this life) for others.