Questions are important. Sometimes more important than the answer. Since John is establishing the foundation of his letter to the community on love, love one another is a major theme. yet the question at the end is also crucial to understanding that John is not seeking an abstract love. John is speaking of love in tangible ways.
As believers we show our love to one another we know what real love is because Jesus gave up his life for us....a tangible sign of love. So when the question comes;"if someone has enough money to live well and sees a brother or sister in need but shows no compassion—how can God’s love be in that person?" This is at the root of authenticity of koinonia (true fellowship/community) for what John is speaking to his community about. Yes we know we are supposed to love one another but this cannot be lip service.
Given the state of refugees in the united states and the response from people given their political views and even some of their religious views, one might ask the same question that John asks. This is not to politicize the refugee issue but it is to ask the question when someone stands up to speak to a child refugee as a christian that "you are not welcome here, Jesus didn't break the law" says something about their understanding of christian love. In response, we might be prone to ask the question John asks - "how can God's love be in that person?" In the Gospel of John, love is poured out for us to see...."For their is no greater love than this than to lay down one's life for one's friends. To build authentic communities, the root of which is God and God's love, we are tasked with the incredible difficult thing to love one another in tangible ways so that the world might know God's very love.
Lord, help us to love other in incredibly radical ways so that your love would shine in our darkness. -Amen