The 'hidden Christians' (kakure Kirishitan) are the descendants of Japanese Catholics who went underground when Christianity was outlawed in Japan during the 17th century. Facing persecution from the authorities, these 'hidden Christians' continued to practice their faith in secret and passed prayers and teachings (containing a lot of loanwords from Latin, Spanish and Portuguese - the language of the missionaries) down orally - more a matter of necessity since most of the 'hidden Christians' were illiterate peasants.
What is interesting though is that in that two hundred year period the 'hidden Christians' was underground, the version of Catholicism many of them practiced slowly drifted away from what you might call 'orthodox' Christianity and became distinctly Japanese in flavor. They lost the meaning of the prayers - which became pretty much like Buddhist mantras: something that only needed to be pronounced correctly without regard for the actual meaning of the words - and their religion generally became a sort of ancestor cult (the ancestors in this case being the martyrs), with a heavy influence from Buddhism and Shinto (the native religion of Japan). Due to the lack of actual priests, lay leaders began to lead the services; these lay leaders, in turn, became more or less the unofficial 'priests' of the hidden Christians, with their 'priesthood' becoming a hereditary position passed down from father to son. The absence of priests also meant that the hidden Christians only preserved one sacrament: baptism.
The only written 'Hidden Christian' document to survive was a thirty-page document from the early 19th century known as Tenchi Hajimari-no-Koto (天地始之事 Of the Beginning of Heaven and Earth). The document comprises of familiar biblical stories (the creation, the flood, the life of Jesus) mixed in with apocryphal material and strung with Japanese elements. Some people might be shocked at just how the stories have become 'distorted' and 'corrupted' (from an orthodox point of view) - sometimes almost veering into borderline heresy - but the work is really valuable in understanding the religious attitude of the Japanese. How the stories were altered during the process of transmission gives a window to the thoughts and sentiments of the people who made them.
(There's an English translation of the Tenchi by anthropologist Christal Whelan: The Beginning of Heaven and Earth: The Sacred Book of Japan's Hidden Christians. Quotes from the Tenchi are from her work.)