Seohak was the introduction of technology, philosophy and most prominently, Catholicism and Western ideas to Joseon Korea in the 18th century.
Catholicism entered Korea indirectly in the 18th century via limited transmission of royal messengers carrying books from missionaries in Qing China.
In this way, Seohak slowly entered Korea in the form of foreign books translated into Classical Chinese.
Although Yi Sung-hun would be released, Kim Pom-u would later be remembered as the first Catholic martyr in Korea after his initial exile and subsequent execution.
Furthermore, the literal reading of the Western religious texts seemed to promote an idea of social equality which challenged the established order at the time.