The unity comes from being in Christ, who is the head of the body. Being in Christ is the thing, and is no temple made with human hands, therefore it can never be a man-made structure, not even Solomon's temple.
Unity was never meant to be one massive organization. That is the lesson of the tower of Babel.
There's a tension between the fallen aspirations of fallen man, which can't see beyond the borders of this world, and the born again aspirations of the regenerated man, which see into heaven.