We agree that Christ established His "church", His body, bride, called-out assembly, and that it most certainly IS one. That "one", however, is NOT the Roman Catholic Church. It goes much deeper and broader than that. Jesus prayed that we would be one, and we are - His prayers will always be answered yes. What some, obviously, need to realize is that this ONE body is already that way in God's eyes. What binds us as ONE is the faith in Jesus Christ - not the various and sundry, imagined and developed doctrines that various and sundry assembly leaders thought up over the centuries.
Your mistake is in not thinking like God does and getting distracted by numbers - not that the Roman Catholic Church doesn't have its own issues with numbers of rites, disagreements with "core" tenets and renegade factions. Our Christian faith is based upon the doctrines taught in sacred Scripture. It is the divinely revealed word of God, so why wouldn't it be the basis? Just as the early church leaders taught and defended the rule of faith by using God's word, so also must all those who came after and, if they cannot prove by Scripture the professed doctrines, then they should be reproved and NOT accepted as teachings from God. Why on earth would someone think God would reveal something privately that contradicted His divine word? Yet, the Roman Catholic faith does so on many points - some so grievous that they caused numerous schisms among believers and sowed dissension and division. For all the hoopla about "we are the true church", you guys sure do have a spotty record on how you have conducted yourselves in God's name. These are the reasons why millions have left and joined to others assemblies who worship God in spirit and truth.
I don't deny that there are many within the Roman Catholic religion that ARE part of the one body of Christ - at least I sure HOPE so. It is because their faith is in Christ and not in their church. NO church can save someone. It is only by grace through faith that anyone can be.
“In the end, the Anglican appeal to a three-legged stool relies on individual interpretation, just as the Protestant appeals to sola scriptura. The three-legged stool turns out to be a theological pogo stick”.
http://www.catholic.com/magazine/articles/how-do-we-know-it%E2%80%99s-the-true-church