The catholic and apostolic [notice no capital C] just means one and united, not the Roman Catholic church.
You're right about most Christian denominations, i.e. Baptists, Methodist and Protestants all being basically the same. I can and do worship in a mix of these, which is cool. However, the Roman Catholics are a different matter. They believe in some fundamental things Protestants don't, for example, they're not allowed to serve a Protestant Communion unless he's in danger of death. [I think I'm right here.] They also have a different Bible to Protestants.
Of course both still believe in Jesus Christ, and will undoubtedly be saved, [in my opinion]. But there are big differences.
God Bless
Nick