I would agree with you on that statement.
However, the debate we are having seems to focus on history, and history cannot be denied, only interpreted.
The “history” of the Church is recorded, documented, and actual and did not start to change until “MAN” changed it during the Protestant revolution.
The church changed quite a bit around 600 AD, especially with doctrinal positions which were built to reinforce a worldly institution rather than based upon God’s Plan.
The "debate" is about the Biblical truth that there is ONLY one church - but NOT the kind you seem to associate with that word. The Body of Christ, the church - which is the called out assembly - consists of all those who have received Jesus Christ, believed on him and been born again into the family of God. They have also been indwelt with the Holy Spirit - who God gives to all who have received Christ. That Holy Spirit is called the "earnest of our inheritance" which means we are sealed with the stamp of belonging to God and he indwells those that are his.
This "history" that you are so confident of is not as cut and dried as you have been led to believe. The Reformation came about precisely because the Catholic Church had left the orthodox faith found in Scripture especially that we are saved by grace through faith apart from works. I can provide you with many links to scholarly articles explaining what happened and you SHOULD know the truth. This link will take you to some of those articles. You owe it to yourself to know what is the truth and not just rely on others to tell you what it is. The link is http://triablogue.blogspot.com/2010/04/historical-roots-of-reformation-and.html. God bless you.