"33,000 denominations"

Much has been made about the "33,000 [Protestant] denominations" argument that every so often makes an unwanted appearance. Recent posts on it can be found here and here. This is not to say that all or even the majority of Roman Catholics think it is a good argument, for there are those who recognize that rampant misuse of a source does not imply that source is insufficient or unclear, in this case with respect to one's spiritual needs (example).

To those, however, who yet remained unconvinced or suspicious that Protestants are pulling some slight of hand, perhaps some perspective might be offered if one were to use the following, analogous argument: suppose an atheist says he disbelieves Christianity because there are thousands of different denominations within it. How would the Roman Catholic respond to this? Much in the same way the Protestant does: he would argue that there is only one perspicuous, final authority to which all ought submit and that those who claim to be Christians but reject this source (or contents thereof) are not truly Christians. The perspicuous, final authority is not to blame for the blockheads who just can't understand that it must be regarded and treated as such. While the identity of the final authority within RC differs from the Protestant's, the method of argumentation is the same.