In this thread, I thought we might expand this question to all religions, as well as to include not just written documents, but also orally transmitted teachings, rituals and practices, etc.
I’m mainly interested in cases where the existence of the secret document / teaching / ritual is undisputed among the experts, so let’s try and keep speculation to a minimum.
I am only interested in religious documents or teachings which were, or are, meant to be kept secret from the public at large. That one fails on both points.
The New Testament Apocrypha kind of qualify… some of them were lost, but we’re not sure. If some parts weren’t lost, then they’re being hidden. Things like the Gospel of Eve, which encourages people to acknowledge that the flesh is inherently weak because god made it that way, so rather than deny themselves pleasure to remove weakness, enjoy life’s pleasures because that’s intelligent design for you.
That “the Scientology lawyer” seemingly never even attempted to deny that the document was Hubbard’s work, as well as that Atack - who I think is widely believed to be a reliable fellow - claims to have seen it with his own eyes, all speaks, I think, in favour of both the document’s existence, and its authenticity.
To my knowledge, “The Blood Ritual” has never leaked (no pun intended!).
How about the Eleusinian Mysteries? Their existence is beyond reasonable dispute, but most of the details were never written down. I’m not sure if you count it if the ritual is no longer practiced, but for hundreds of years the major secrets were kept.
Considering that the Apocrypha/Deutrocanonical books have been published in both Roman Catholic and Protestant Bibles, I don’t think they fit the OP’s question.
Mandaeism has some secret texts unknown to outsiders, assuming I understand wiki correctly. This gnostic faith has some 65,000 followers and has been studied in broad outlines, with some of their texts published in the early 1900s. Before 2003 they mostly lived in Iraq. Following the chaos of the US invasion most moved elsewhere: Iran was a favorite destination as it already had a community. Mandaeism - Wikipedia
Plenty of stuff from the old mystery religions is still secret, and we can only speculate about it. Very few documents about the practice of Mithraism have survived, for instance, although we have a lot of artwork. (I think that David Ulansey’s ideas about Mithraism being a science-inspired “new” religion which only borrowed the trappings of Persian Mazdaism is likely correct, though.)
We don’t know everything about the Eleusinian mysteries, either:
Or similar mystery religions.
It’s not a religion, but the secrets of the Skull and Bones society are still secret. There are plenty of other college secret socieities in nthe US:
…and nobody not in the fraternity knows what’s in the Colgate University Deke* Temple.
Delta Kappa Epsilon, which has plenty of branches elsewhere. Attached to the Deke house at Colgate is a windowless “TEmple” that outsiders don’t get into. Speculation about its contents and activities therein are common, ranging from Lovecraftian Esoteric Order of Dagon ideas to the claim that it contains a swimming pool filled with wine.
Worth a mention are the Kakure Kirishitan of Japan or the hidden Christians. Yes, there are 2 wiki pages. Christianity was outlawed in Japan with severe penalties from ~1639 to ~1873. While underground, Christianity began to take Buddhist aspects: the tradition was oral since Christian documents could get practitioners in trouble.