Those who follow Jesus actually don’t belong to this world. Jesus says exactly that in John 15. Peter says we’re citizens of heaven. Both Peter and Paul say we’ve become a part of the family of God. But we still live in this world and we have to deal with decisions, tasks, pressures, and ideas while we’re here. So in John 17:14-18 when Jesus says that He has sent us into the world, but that we are not ‘of the world’, it means that we need to understand how to live as people who think differently, believe differently, and live differently while interacting in our mission field for Jesus.