You are right for a lot of reasons.
1 Cor. 12:13-14 "For by one body-whether Jews or Greeks, whether slaves or free-and have all been made to drink into one Spirit. For in fact the body is not one member but many."
The body is all those indwelt by the Holy Spirit. IOW, all believers are united together, not by the church they are a member of here on earth, but by the church they have been sealed into by the Holy Spirit.
Also, if you look at the development of Christianity you do not find the monobishophoric system until at least a couple generations after the Apostolic era had ended. IOW, the idea that there was always just one church operating under a hierarchal system is a myth.
You're right. Wish we could convince the RC's of their error but that will have to come from the HS. We can't do it because they've been indoctrinated about these heresies since childhood. I'm sad for them because they so often miss the mark.