Kakure nenbutsu

Kakure nenbutsu (隠れ念仏), or "hidden Amida Buddhism", was a form of Jōdo Shinshū Buddhism secretly practiced on the Japanese island of Kyushu, in the Hitoyoshi Domain and Satsuma Domain, during a period of religious persecution from 1555 to the declaration of freedom of religion during the Meiji Restoration. Because it became a secret lineage, some "kakure nenbutsu" lineages continued into the mid-20th century.
Origins.
The "kakure nenbutsu" era began with the "ikkō-ikki", violent uprisings of peasants encouraged and organized by the leadership of Shinshū splinter group Ikkō-shū. The "ikkō-ikki" were part of a period of political instability in Japan and the threat they posed to leadership was very real. In response to the uprisings, in Kyushu all forms of Shinshū were banned. 
Secrecy.
Some peasants who believed in Shinshū began meeting in secret, usually in mountain caves some distance from any inhabited area. Others would make secret pilgrimages to temples in neighboring domains.
Persecution.
"Kakure nenbutsu" continued into the Edo period and were persecuted as secret organizations. In one instance, 1,700 believers were arrested. However, believers remained quite faithful and would send donations in secret to the Hongan-ji temples in Kyoto. Some members would appeal to Hongan-ji to help them with preserving orthodoxy.
Survival.
Some "kakure nenbutsu" lineages departed from orthodox Shinshū and merged with yamabushi and other secret, esoteric practices. These groups, called "kayakabe", continued to survive as of 1999, although there were virtually no young members.
The term "kakushi nenbutsu" (隠し念仏) refers to a different type of "hidden Amida Buddhism" practiced in the Tohoku region. Unlike "kakure nenbutsu", "kakushi nenbutsu" still exist today as unregistered religious groups.