I think the thing to keep in mind is that it is a *Great Mystery* (in St.Paul's words) and refers to the already-and-not-yet reality that the Church is the Bride of Christ. The union of husband and wife is a sacrament (outward sign) (symbol that effects what it signifies) (magnum mysterium)of Christ and the Church.
So the Church is also the Body of Christ, because Christ and His Bride together make One Flesh. And thus it is also Incarnate, not "just" spiritual, because Christ is not just spiritual: he is embodied. He is incarnate and will always be incarnate, forever.
So it cannot just be an "invisible church." It has to be a "visible church" as well. The incarnate one we can see.
And it has to be a "visible church" the believer can find ("Go to the street called Straight) ("let him take it to the Church") and which speaks in a way he can hear ("...and if he refuses to listen even to the Church, regard him as you would a pagan or a tax collector.")
And this visible Church Christ has joined to Himself, is joining to Himself, and will join to Himself in fidelity: He cannot be separated from Her. He indeed identifies with Her. What you do to Her, His Church, His Bride, you are doing to Him:
"Saul, Saul, why are you persecuting Me?" "Who are you, Lord?" "I am Jesus, Whom you are persecuting."
This becomes all the more important, the more the culminating days of confusion and apostasy approach.
"And let us consider how to spur one another on
to love and good deeds.
Let us not neglect meeting together,
as some have made a habit,
but let us encourage one another,
and all the more as you see the Day approaching.
It is truly craven to leave the endangered Church, the visible Church, the Bridal Church at just the time when we most need each other. We need that "spurring on" to love and good deeds; we need that encouragement to keep on believing Christ's promise that He would not leave us orphans but would always be with us, all the more when danger most prevails, and we see approaching the great and terrible Day of the Lord.
The Body of believers IS the ‘visable’ church regardless of where they gather together.
Jesus said.....”For where two or three gather together in My name, there am I with them.”.....matt. 18:20
Every day two or three, or many more, may be assembled in every city or village in the US, in England, in Greenland, in Africa, in Ceylon, in the Sandwich Islands, in Russia, and in Judea in almost every part of the world, and in the midst of them all is Jesus our Saviour. Millions even at the same time, in every quarter of the globe, worship in his name, and experience the truth of the promise that he is present with them. This ‘IS’ the Body of Christ..the Church.