Many RC's think that theirs is the only 'true' church.
That is not a mistake that only some RCs believe in. God has no bounds, plain and simple.
It's like humans thinking that there could be no other life forms out there in the universe, and yet many of all cultures and levels of education buy into this as well. How can only Earth have life, and that we as humans are the only intelligent life that exists in the whole universe which is infinite? Again, whether you are religious or not, why put boundaries on creation itself and be so self centered to think that only humans are capable of evolving into the only intelligent life-form to exist?
What I'm about to say in this post needs no response. I intend it as food for thought, for you to reconsider your criticism.
The RCC teaches that the RCC is the one true apostolic successor to the Church as lead by Peter, and that the Bishop of Rome is the direct successor of Peter.
When they say they are the one true church, they are stating something that every denomination states: that their doctrine/interpretation of the Gospel is right, and the closest to the intention of God. If you think that all denominations are equally right, then why do you follow your denomination? If it is no better than any other, then every other will do. And if all denominations are equally right, how do you reconcile such a concept, with the contradictory concept taught by most, that they are more right than your own.
Take, for example, the concept that has been proposed in this thread: that all sin is equal in the eyes of God. The RCC and many other denominations reject this concept. Others embrace it. They are contradictory, and only one can be right.
This is a weak criticism of the RCC, when there are real criticisms to be had. It is mere theological relativism. And relativism is merely a well-dressed nihilism.