I think this relates to the fact that God, having made His world in a “state of journeying to perfection” as the Church teaches, has so arranged things that man participates in his own perfecting, in completing His work so to speak. In this way we become more like Him as we come to “own” it, as we come to increasingly align our wills with His perfect will, with His help. And as the kingdom is realized within ourselves, as we’re reconciled with God, animosity against and strife with neighbor decrease as love replaces it and so oneness grows in each one of us at a time regardless of whether or not anyone else cares. Either way, yes, we need to promote and encourage this unity but it’s realistic only within the context of religion, involving man’s relationship with God first of all. Hippies tried and ultimately failed miserably at it with their own humanistic social experiment.