The case of the "Kakure Kirishitan" (lit: hidden Christians) in Japan is an interesting case. Essentially adherents to the Catholic faith during a period of persecution by the Buddhist/Shinto/etc in the 1600s.
They went so far as representing icons such as the Virgin Mary holding the baby Jesus with statues of what appeared to be Buddhas and Boddhisatvas in the same poses.
This variant ended up (as you might expect) slowly becoming something very distinct from traditional Catholicism. In general they merged back into the mainline Catholic faith when it was eventually permitted, but some remained separated, creating some really fascinating religious syncretisms.
That also reminds me of not hidden Lithuanian Tatars. Basically Tatars living in Lithuania for centuries. While mostly cut off from the rest of the Muslim world never adopted Catholicism.
