When Jesus tells His disciples that they're not of this world, He never told them not to be in this world. We're on this earth for a reason, and we can't run away from the sin and struggles that exist because of the broken nature of humanity. It's important, though, that we allow ourselves to be filled with God and His Holy Spirit before we walk out the door every day. When we do that, we're less affected by the world, and better able to affect the world with the eternal hope of the Gospel.