Mystery religions that I refer to are those such as found in the Roman Empire around the Middle East before and during the early current era. Examples include the Eleusinian Mysteries, Gnosticism, Mithraism and Pythagoreanism. A present-day mystery religion is Scientology. They all comprise of a series of outer mysteries and public rituals. Sometimes these would be public holidays, sometimes they would cause regular social upheavals (leading to the Romans suppressing various mystery religions). The Egyptians had a similar structure with cults, many had public parades and displays. These outer mysteries are superficial, attention-seeking methods employed by religions to attract recruits who are interested, beguiled or intrigued by the symbolism used during them.
“Each Mystery tradition had exoteric Outer Mysteries, consisting of myths which were common knowledge and rituals which were open to anyone who wanted to participate. There were also esoteric Inner Mysteries, which were a sacred secret known only to those who had undergone a powerful process of initiation. Initiates of the Inner Mysteries had the mystical meaning of the rituals and myths of the Outer Mysteries revealed to them, a process which brought about personal transformation and spiritual enlightenment.”
Neophytes who approach the religious group are educated and taught the Inner Mysteries. Frequently, the Outer Mysteries would comprise of re-enactments of basic astrological stories of the fall and rise of the sun, and of stars: of gods. A popular one is the three day mourning on the Winter Solstice of the death of the sun, followed by a rejoice at its birth as the days begin to get longer around the 25th of December. The Outer Mysteries would be public displays of mourning then rejoicing, and the Initiates would be taught what it all means, like how today many Christian parents will teach their children about the 'real meaning of Easter'2, which is the equivalent of an Inner Mystery part, whereas the chocolate, eggs and festivities are the Outer Mystery, public component.
A religion was a Mystery Religion if only the initiates were allowed to know the keys with which to decode the Outer Mysteries. It is an early religious method to take theological concepts and encode them into stories, creating characters and events which need to be "decoded" in order to see what the storyline actually means. Not knowing the key would be like reading "Animal Farm" by George Orwell3 and not knowing it was about Communism, or attempting to read Finnegan's Wake literally! The keys, the inner understanding, allowed initiates to see what the story was really saying. Such stories were created to be fascinating, a little compelling, so that certain types of people would be attracted to the group through the symbolism used in the story, even though they don't actually know the inner meaning.
All religions can appear to have 'external' symbolism. Almost any religious statement is understood differently according to the depth of knowledge of the observer. A trained Christian theologian understands the basic Christian dogmatic statements vastly different from the average person on the street. A politician versed in history will be reading a completely different version of Animal Farm3 than will a school kid who has picked it up for a general read. How can we differentiate between any field of knowledge and a mystery religion? They both appear to have one shallow external reading and a deeper, unlocked, meaning for those initiated into the correct area of thought.
Another example can be found in GCSE physics. Students are taught that electrons orbit the nuclei of atoms. Those who are more fully 'initiated' into the "inner mysteries" of basic physics will of course learn that electrons do nothing of the kind. The outer statement is meant as an introductory teaching that is easy to understand for neophytes, and the "inner teaching" is more complex, but is best understood after first accepting the outer teaching. To teach students complex subatomic physics, quantum mechanics and probability areas would be a quick way of ensuring that no student ever learns anything! So how can we differentiate between special fields of knowledge and a Mystery Religion?
One differentiation is intent and context. George Orwell was a well educated and accomplished writer and encoded stories within more superficial stories - making even the cover story a compelling and exciting one - however, no society of initiates has built up around Orwell's ideas found in the book, and there is no secret passing on of keys or ideas which the general public cannot accept or understand, his books were one-offs, and the "key" to unlock the stories deeper meaning is found within politics and history. As we do not consider politics or history to be Mystery Religions, the keys are public, so it is not a Mystery Text. Likewise with science - the "keys" to decipher any scientific statement are readily available in printed books and via experiments, they require no acceptance of religious or dogmatic statements.
What is common in Mystery Religions is an intentional withholding of information about the finer picture, or a withholding that intentionally filters out those who are not convertible. The symbolism employed must attract those who will accept the teachings, and put off those who do not. And this brings us to a point where I can propose that Satanism acts as a modern Mystery Religion.
Satanism can be said to have intentional Outer Mysteries that are understood only by elite individuals who are granted intellectual understanding according to the will of Lucifer4. The Outer Mysteries of Satanism include our own music, art, film and creative outlets, much of which does indeed feature a personified form of evil, Satan. The Inner Mysteries teach that there is actually no such being, but that Satan represents the carnal nature of man, and the dark force of nature. The Outer Mysteries contain religious trappings that attract certain types of people, but repel most.
Note that many of these are as portrayed by pop culture, not by Satanists themselves, but nonetheless these portrayals act as the Outer Mysteries of Satanism. Satan, the scapegoat, gets the full whack of public paranoia against 'others'.
“Almost all that the CoS has in common with what people might expect Satanism to be is the sinister mask its founder and members like to present to the gullible. [...] The performance is supposed to disturb, perhaps even offend, people - but it is play acting. LaVey's Satanism behind the mask and masquerade is something else: a "self-religion".”
Graham Harvey (2009)5
So what are the Inner Mysteries of Satanism?
When considering how to differentiate between special fields of scientific knowledge and Mystery Religions, I noted that science is not a Mystery Religions because "with science - the "keys" to decipher any scientific statement are readily available in printed books, they require no acceptance of religious or dogmatic statements.". It could easily be noted that The Satanic Bible and Satanic books and websites clearly explain the materialistic, iconoclastic and left hand path teachings of Satanism, so this open access to the Inner Mysteries could be said to disqualify Satanism as a Mystery Religion.
However, initiates into Satanism are a different class of people to mere "understanders". Many atheists, non-religious have looked into Satanism with an open mind and concluded that the teachings are legitimate, a sound philosophy, but that they don't like the word "Satan" or the imagery. Sometimes they find it pointless, immature, needlessly provocative, etc. These are people who have understood the philosophy, but not become initiates or disciples of Satan, and have not accepted Satanism. The symbolism repels them, the Outer Mysteries repel them.
Without being drawn to the Outer Mysteries as well as accepting the Inner, it is unlikely a person will call themselves a Satanist. This is a result of Satanism operating as a Mystery Religion, with an external set of rites and symbolism that deflect most people, and only allow a few into emotional initialisation. The philosophies and teachings themselves are readily accepted by many down-to-Earth people. Some people enjoy dogma and religious trappings, whereas as many do not. Many who do not convert to Satanism are put off by its imagery and symbolism, not its teachings.
Satanism is a Mystery Religion in that inner understanding and teachings are only readily available to those who are initiates. Its symbolism and public-facing side are designed so that many will automatically reject Satanism and not wish to examine its beliefs: So although its beliefs are public, laid out in The Satanic Bible6 and other writings of Satanists, its teachings are simultaneously kept secret by a veneer of symbolism that puts off anyone unworthy of understanding. Although Satanism may not be a 'mystery religion' in the full or classical sense of the term, it effectively operates as one.