When the West first established trade relations with Japan, one of its major exports, alongside goods, was religion. Brought in by foreign missionaries, Christianity began to catch on—especially around Nagasaki, the port that became Japan's key link with the Western world. Fearing Christianity's expanding influence, Japan's rulers banned the religion and persecuted its followers—but rather than abandon their faith, many of Japan's Christians went underground, practicing in secret for around 250 years.