Catholics believe THEY are the only true church. I can't change their opinion but it's just an opinion. God's church is the entire body of believers, not a structure, not a denomination.
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.
Luther, Zwingli and Calvin turned their backs on the Church and set up their own, paving the way for the creation of multiple churches. The mainstream Protestant churches - the Anglican, Presbyterian, Baptist etc. - were also found wanting and thus the American Restoration movement was formed, which created Disciples of Christ, Churches of Christ, Independent Christian Churches/Churches of Christ, The Church of Jesus Christ of Latter-day Saints, Seventh-day Adventists, Jehovah's Witnesses, and others.
Now we have the New Testament types who are increasingly independent in forming their own theologies.
All completely independent from each other. All differing either slightly or grossly from each other. The Church that Jesus left on earth had the authority to ensure doctrinal purity. Acts and the Epistles document extensively the authority that the Apostles and the early Church officials had to deal with the individual church communities across the known world in correcting, instructing, admonishing and directing them.
There is no independence in God's church. It's not the rules of the individual. It's His rules. We cannot make it up as we go along.