While some of the comments are getting into some of what sounds like rather deep theological discussion and quoting of documents, etc, I'm mostly disgusted by the swiftness with which people take offence at "I know that my church is right and yours is wrong."
Allow me to explain.
I know that the earth is round. Have I actually flown around it, or do I completely comprehend the scientific explanations of how we know it's round? No.
But I do believe that it is round based on others evidence and explanations. And if someone were to come up to me and insist that it is flat, I would have to shrug my shoulders, and say, "I'm sorry, but I believe you are wrong. The earth is round."
I also believe that the Catholic Church holds the fullness of Truth. As a matter of fact, I believe that I have more *personal* evidence of that than that the world is round!
WHY would it be offensive for me to tell that I believe that to someone who didn't agree with, that statement?
I suppose the difference would be that the earth being round is an acutally scientifically provable point, perhaps the Truth of the Catholic Church isn't.
ok, so what if I try the example of something a bit more arbitrary? (I admit, I haven't totally thought this through, I'm kind of just typing as I think :-)
What if I said, "I believe that my recipe for brownies tastes better than your recipe. While I admit that it wouldn't be the nicest thing to say, taken right out of context, but let's say you and I were trying to figure out which recipe we should enter in a contest? A very important decision! Then why would it be wrong for me to assert that my recipe is the best? It would certainly be necessary to the conversation!
If there were an even more important decision to make, then why I should I not make the statement, "I believe that [this particular statement,] which you hold to be only an opinion of mine, is factually the highest truth."?
And that is, of course, what Catholics are saying when they say that their Church is "completely right" or something to that effect. (sorry, this is where my lack of "theological terminology" comes into play!)
Coming at this from another angle; I have often wondered why more non-Catholics don't make the same claim: "My religion/church holds the fullness of salvation/truth."
I would not want to belong to a church that was "not sure" if it was completely right, at least in the main doctrines.
And if I myself wasn't sure what I or my church believed about any of the main disputes among Christians?
well, I would certainly not stop seeking until we had figured that out!
and, to flip it around? If someone were to say to me, "I believe the fullness of truth is found in my church." I'd say (or think,) "well, that seems a bit arrogant, but not really, cuz I actually think that same thing about MY church, so obviously, *one* of us is wrong..." but I'm not going to be like offended... (maybe cuz...I actually KNOW that I'm right? :-D )