"The universal (small "c" catholic) church is invisible, because the ekklesia of God are only those in whom the Holy Spirit resides. Only God knows those who are his."
How can the world see an invisible unity? John 17 says that the end of this unity is that the world may believe. How can it believe in what it cannot see?
What it cannot see??? Individuals are not invisible. "The true church (ekklesia)" is made up of individuals in whom dwells the Holy Spirit. When these regenerate individuals (the wheat)congregate together as a "church organization" in a building, they are joined by tares (unregenerate). Only the "visible church" is made composed of the wheat AND the tares. They both look identical (outwardly) and God allows them to "grow together", until he returns and divides them.