"Catholics are not Christian"
This laughable attitude is not uncommon among Korean Protestants. Sadly, I've even had Catholic students say that they were "Catholic, not Christian" and not realize what they were saying.

Here is the entire text of an article, Religions' viewpoints, that explores this:

Christians in Korea are no different than those elsewhere


Here's this week's tip on Korean language and customs:

Q:
Something notable about my stay in Korea has been the proliferation of churches not only in Seoul, but nationwide. However, recently I discovered something a little disturbing.

In a conversation with a Korean Christian, I inadvertently let slip that I had been brought up as a Roman Catholic, although I no longer practice the religion. The person I was talking to informed me that Catholics were not Christians, which needless to say dumfounded me as the Church of Rome is the oldest Christian religion today.

What is the cause of this viewpoint and is it common in Korea?

A:
The issues concerning the distinction between Roman Catholicism and "Christianity," which have continued for centuries around the world, are nothing new in Korea.

Since both Catholicism and Protestantism were introduced in Korea a long time ago, Koreans, who have embraced either or both, have pursued their ideologies, depending on which they belong to, the way others might do elsewhere in the world.

The particular reaction displayed by the Korean Christian mentioned above is not necessarily considered unique to most Korean Christians, but personal.

In Korean, the term 기독교 (基督敎), pronounced Gidokgyo, means "Christianity" and comes from the Chinese characters 基督, meaning "Christ" and whose Sino-Korean pronunciation is Gidok, plus the character 敎, which means "teaching" or "doctrine." In practice, however, this term is generally used to refer only to Protestantism, which in Korean is properly called 개신교 (改新敎), prounounced Gaeshingyo and meaning "Reformed Teaching."

Catholicism is traditionally designated by the term 천주교 (天主敎), prounounced Cheonjugyo, which literally means "Heaven Lord Teaching." In Chinese, the first two characters are pronounced T'ien chu and mean "Lord of Heaven." This was the Name applied to the True God by Matteo Ricci, S.J. (Lì Mǎdòu - 利瑪竇), the great missionary and scientist to Ming China.

Recently, the terms 그리스도교 (Geuriseudo-gyo) is sometimes used to mean "Christianity" and 카톨릭교 (Katollik-gyo) "Catholicism." Interestingly, these terms avoid Chinese characters except for the final one, 敎, meaning "teaching." They respectively employ the Korean transliterations of the Greek "Χριστóς" (Khristós) and the French "Catholique."