The unbreakable bond between love of God and love of neighbour is emphasized.
One is so closely connected to the other that to say that we love God becomes a lie if we are closed to our neighbour or hate him altogether. Saint John’s words should rather be interpreted to mean that love of neighbour is a path that leads to the encounter with God, and that closing our eyes to our neighbour also blinds us to God.
– Pope Benedict XVI, Deus Caritas Est (God is Love) #16