Well that's definitely the right criticism to bring to it, and it's what I've wondered about as well.
The book, if you take a look at it, uses explicitly the marxist framework, ultimately to prove that Socrates was a right winger in his later adulthood and that he was executed for his part in the tyranny of The Thirty. (the semi-marxist author clearly loves Socrates, so it's a strange and fascinating read)
That's why I say it's an interesting theory, and it's worth taking some of its points into account because the author gives some wonderful details and argues well, but certainly it is not definitive.
Has an underclass ever done what you describe? I'm not sure. Certainly certain cults have sprung up from underclasses before, particularly when they are not integrated into the society fully or are excluded in some way, left on their own to carry on their worship. look at India, as many strange cults and even paths promising immortality or godhood have seemed to come from at least relative lower classes particularly after the Vedic period.
For the example of this book, the author doesn't make it totally clear how he thinks it happened. Was it originally a mystery tradition of a priestly or warrior caste with a different original import that was seized upon by the downtrodden when the tribal order broke down, and quickly bastardized by them? Or was it some other kind of development more original to the rites of the lower classes in his view? And in reality, was it any such thing or a priestly continuation of noble rites which had a noble esoteric content?
He does claim it comes originally from Demeter cults, the grain of wheat being a symbol of rebirth/immortality. So as long as the lower classes were part of the Demeter worship, it could have come from a development of that idea. You may postulate a division between IE derived and pelasgian derived traditions which manifested as elements of a class struggle, rather than saying its purely based on class.
Isn't Christianity itself basically a mystery religion developed (via borrowing and bastardization perhaps) by an underclass, or at least by priestly types of an oppressed ethno-religious group wanting to cater to or manipulate the sensibilities of the downtrodden among their countrymen? It isn't initiatic or it doesn't go beyond mysticism, but it obviously has elements similar to orphism, mithraism, buddhism, etc., if in a degraded form.
In the end what this "marxist" narrative accounts for, which must be accounted for, are the elements in Orphism that may tend toward universalism, toward the democratization of immortality and the soul, toward the hostile opposition of soul and body and the hatred of the body, toward the foreshadowing of many values we would later find in Christianity.
As Spengler polemicizes:
"We know the tremendous formula of Orphism, the Nay of the mysteries that answered the
Yea of the agon, which arose, certainly by 1100 at latest, as a protest of Waking-
Consciousness against Being — awjua criixa, that splendid Classical body a grave!
Here man is no longer feeling himself as a thing of breeding, strength, and
movement; he knows himself and is terrified by what he knows. Here begins
the Classical askesis, which by strictest rites and expiations, even by voluntary
suicide, seeks deliverance from this Euclidean body-being. It is an entirely
erroneous interpretation of the Pre-Socratics to suppose that it was from the
view-point of enlightenment that they spoke against Homer. It was as ascetics
that they did so. These "contemporaries" of Descartes and Leibniz were
brought up in the strict traditions of the old great Orphism, which were as
faithfully preserved in the almost claustral meditation-schools — old and famous
holy places — as Gothic Scholasticism was treasured in the wholly intellectual
universities of the Baroque. From the self-immolation of Empedocles the line
runs straight forward to the suicide of the Roman Stoic, and straight back to
"The Apollinian religion venerated body,
the Orphic rejected it, that of Demeter celebrated the moments of fertilization
and birth, in which body acquired being. There was a mysticism that reverently
honoured the secret of life, in doctrine, symbol, and mime, but side by side with
it there was orgiasm too, for the squandering of the body is as deeply and
closely akin to asceticism as sacred prostitution is to celibacy — both, all, are
negations of time. It is the reverse of the Apollinian "halt!" that checks on
the threshold of Hybris; detachment is not kept, but flung away. He who has
experienced these things in his soul has "from being a mortal become a god."...
"As the songs of Achilles and Odysseus were dying
down everywhere, a grand, strict doctrine arose at the famous old cult-places,
a mysticism and scholasticism with developed educational methods and a secret oral tradition, as in India. " - Decline of the West Vol. II
I think ultimately it may be important to make the distinction between mystery religion and initiatic order a bit clearer.
And if you can inform me better on the details of the rituals and initiations/mysteries to help me get a better picture of what it was all about, that would be very appreciated.