Understudied phenomenon

Fellow translators,

I'm not looking for help with Spanish, while I'm translating a book from English to Polish. And I haven't found better place in a short time to ask a question. I just need someone, whose mother-language is English. Or at least someone brigther than me in translating from English.
The book concerns the collapse of Eastern Christianity. Everything was just fine, when I came over this sentence:

"A religion that dies in one area, in the sense of having no known active followers, may have many covert or clandestine believers, and - rather like the collapse of churches - crypto-Christianity is a thoroughly understudied phenomenon."

'understudied phenomenon' - what does it mean? Understudy means playing as a substitute for an actor or similar actions connected with theatre. I found one translation, meaning: subsitute. But still, I can't catch the meaning of whole phrase. Does it mean that crypto-Christianity phenomenon occurred in history more often than collapses of the churches? So, it was substituted, understudied by next cases? I was looking for usage of this phrase in different texts. As before, no idea.

Sorry for making a new thread. And I hope you won't kill me for having nothing to do with Spanish.

Understudied phenomenon - help!