That is the position of the Patristic church. Zizioulas says it even more emphatically. Eucharist--the communion event--constitutes the church. It is not an act of a pre-existing church.
But for Ratzinger, the meaning of communion is to become like Christ. It is tied to history and the imitatio Christi--how we will live our life in this world in relation to others. It is how we experience and understand Christology.
For Zizioulas, communion is the action of the Holy Spirit and a preview of the future eschata--our life at the end of history. We are defined by who we will be and not through some moral attainment. This is the realm of Pneumatology.
And perhaps the benefit of ecumenism is a synthesis of these two approaches.
Now that is an excellent observation in almost all its parts! I say almost because I fear that any "substantial" focus on the world will, given our nature, lead us to become so concerned with things of this world that we could end up defining the Faith in terms of what the world sees as important. The meltdown of ECUSA and the abuses and innovations which infect elements of the Roman Church at least in the First World today are object lessons in this. How do we inculcate the "imitatio Christi" if indeed by that we mean a type of social concern and at the same time avoid this?
"This is the realm of Pneumatology."
My favorite, if most confusing, realm of the Faith.
That was short. And exceedingly sweet in its truth.