Christianity was banned in Japan in 1614. For the next 250 years, the Kakure Kirishitan (hidden Christians) worshipped in secret.
The Jesuit missionary Francis Xavier arrived in Japan in 1549, and got to work converting the Japanese to Christianity. Many local daimyō (feudal lords) converted because of the role Jesuits played in trade with the Portuguese, and Christian worship filtered down to their subjects. Within thirty years of Xavier’s arrival, the number of Japanese Christians is estimated to have been around 130,000.
All that changed with the reunification of Japan under the Tokugawa shogunate. Christianity was first discouraged, and then banned completely. Suspected worshippers were asked to step on special pictures of Jesus or Mary (called fumi-e) to prove their innocence. Those who refused to recant were hung by their feet in a hole or well (ana-tsurushi). Twenty-six Christians were famously crucified in Nagasaki, one of the centres of the religion in Japan.
Japanese Christians went underground. Deprived of priests and formal structures of worship, they practised in secret, in private homes with lay leaders. Statues of the saints and Mary were disguised as statues of Buddhist deities, as in the picture at the top of this post. It looks a lot like Avalokiteśvara / Kannon / Guanyin (the embodiment of compassion) but is thought to also secretly represent the Virgin Mary – hence the name Maria Kannon. Other statues had crosses hidden on their backs.
Buddhist chants disguised Christian prayers. Without bibles, the worshippers passed what they remembered down from parent to child. Inevitably, their Christianity mutated and changed in the dark: it picked up a whole range of unorthodoxies and sometimes began worshipping the martyrs as “ancestors” of the secret church. When the Meiji Restoration dropped the ban on Christianity in 1873, some of these secret worshippers came out of hiding and reestablished contact with the Roman Catholic Church. Some held on to their local practices instead, but most of those have since died out.