More than the Catholic Church believe themselves to be the one true Church. There has probably never been a new denomination or independent congregation founded that didn't think they would be the one group that would get it right or at least the most right. And some of those believe they did. Most probably accept Christians in other denominations as fully Christian but not all.Why all the malice when someone claims his/her Church is THE Church?
And it is mostly Catholics saying that. That's because the Catholic Church IS the Church Christ founded. Humans have muddied it up a bit, but does anyone believe Jesus didn't see that coming? LOL
He knows all about humans. He was one.
In my opinion God is far more interested in our relationships with Him and how we treat each other than He is interested in our theology which I think He pretty much tolerates more than endorses. I suspect when we meet in Heaven we'll all have a good laugh over how much of this we got wrong.