I believe that the best response would be as I approach people like this is the following…
What do you mean by that?
This will get you information as to the source of that thought. Was it told to them, read by them or is it a tract.
What do you mean by Catholic?
This give you insight as to what their perception of what Catholic means.
When is that the Church became Catholic as you understand it?
This will guide you into whatever teaching they have. If it was not the early Church, there is a Catholic Church…when do they think it became Catholic?
The purpose of all these questions is to do what is called “chunking down”. Based on general Semantics everyone has a map of the world and in this case it includes the Church. Based on how the mind works and filters information asking what and how gets you where you want to go. By asking questions you get insight into the map that this person has in their head. As you chunk down you ultimately get down to where they got this notion, how much emotion is tied up in it and how best to deal with it.
The other thing I would do is ask what questions as to their belief. My favorite way to do this is to say something like this…
It is my understanding then that you are a Christian?
If not denominational then I ask again this…
What brand or type would that be?
This usually gets a gasp and a HUH? What do you mean?
You then answer…well and it is important you order them as they came into being…there are as you know Catholic East/West, Oriental Orthodox, Eastern Orthodox and Protestant.
If you get back…well I am not a Protestant…then you say well let me ask you this…
Do you believe that the bible is the sole rule of Faith?
Do you believe that you are saved by Faith alone?
Usually you will get “yes” to both…you then can explaint their Protestant beliefs and aid them in understanding where their beliefs came from…
What you are doing is reordering their disordered map of the world and in particular Christian History.
In order to make any headway when confronted with someone with scoffs like this it is important not to respond as you did and go looking for an answer to rebut. You need to know where they got this misinformation.
That is my two bits.