That would certainly be a serious mistake in any RPG, and is a very serious accusation of ArM5. I am not aware of any particular guilt of ArM5 in this respect, though.
Perhaps David Chart's last contribution From the Editor in subrosa #16 helps to sort out your accusation?
Cheers
Last time this issue was raised in resulted in multiple pages of personal attacks on the person raising the issue, followed by the thread being closed with Atlas making the statement that factually Ars Magica does exactly that to a degree. I suggest we not repeat that.
How is Lucretius relevant to medieval magicians? The man wrote in Republican Rome and his work was so forgotten and irrelevant during the middle ages that it was presumed lost until early humanists stumbled upon a copy of his text during the Renaissance. Even if one presumes that the Founders similarly found a copy, De rerum natura is not at all magically oriented and seems a very odd choice for the philosophy of wizards.
But it is an awfully good explanation of why the magic is called "Hermetic Magic" and the society called "The Order of Hermes". If the order simply descended from Roman "Mercurians", why not call it "Mercurian Magic"? Greek was virtually unknown in the Latin West in the 8th century after all. Besides, adopting the name of a pagan god was hardly a politic move at that time, so one presumes there was an ideological motive. Hermeticism explains the situation better than anything else I can think of.
In Mythic Europe the "Cult of Mercury" is a known product/feature of the Roman Empire (known amongst the right kind of scholar, anyway). So, Bonisagus wouldn't want to call his new magic "Mercurian" precisely because of that: Greece is a long-dead civilisation, that was not militarily active in the west, while Rome has "recently" dominated the entire world.
Ostensibly acting like you are reviving the cult of military magicians that powered the Roman Empire, doesn't look politically smart when many of the traditions you are trying to recruit/assimilate derive from druidic/Germanic cults that fought against Rome.
Lucretius is a good, easily verifiable example of a writer, who uses an invocation of a pagan god for literary and conventional reasons, without himself adhering to pagan religion. It thereby also defeats attempts to conclude, that a Hermetic magus, because he uses 'pagan idolatry' (see catnip & garlic) in a spell or book, would have to adhere to a specific religion.
I addressed that already in https://forum.atlas-games.com/t/the-break-room/102/1 . There is no use in speculating about author's intentions, once these are written out clearly.
In ArM5 Hermetic magic is - see HoH:TL p.5 - based on the traditions of the Cult of Mercury, and no mention of Hermeticism is made in this context.
Cheers
So according to this version of the story the use of "Hermes" is just thinly-veiled concealment of "Mercurian" origins, which went over the heads of the wild Germans while the Latins could give each other nod-nod-wink-winks, knowing that the Romans associated Hermes and Mercury?
One could certainly postulate this. I prefer an alternative where Hermetic Magic has some relation to the actual occult tradition also using the name of Hermes. Otherwise one has to explain the existence of the Hermetic Corpus as purely coincidental. It's really stretching it to suggest that a real world tradition which has inspired occultists for centuries and the centerpiece order of magicians in Mythic Europe have a name in common but nothing else. Why would this make the setting better?
I agree with this. I didn't see what you were getting at but certainly practitioners of Hermeticism in the Renaissance considered themselves to be good Christians.
There's nothing in HoH:TL that precludes Hermeticism and your post is citing the 2e book which does in fact bring Hermeticism into the picture.
Historically, there was no connection made between Mercury or a "Cult of Mercury" and magic in the Roman Empire. To my mind, it's a much better fit for the setting to suggest that the Ars Magica "Cult of Mercury" was a manifestation of ongoing Hermetic traditions than to invent a magical "Mercurian Order" with no connections to real tradition or history.
I thought most magi would be whatever religion sponsored the stories most enjoyed by the troupe whose game they showed up in. 
More seriously, most magi would probably practice either an eclectic version of their original religion, or their master's, depending on how old they were when they were taken. Very few magi truly focus on the Divine, and most of those become Holy Magi, who are mostly shunned by the larger Order.
Hermeticism is a possible tradition for a Hermetic magus. It is just not the Religion of Magi in Mythic Europe.
By ArM2 pg. 28 this evidence was unearthed by Seekers - i. e. magi who well after the founding of the Order of Hermes investigated the past. This shows how to integrate Hermeticism into an ArM campaign. But given the secrecy of most Seekers (see e. g. HoH:TL p.15), their findings are not widely known.
ArM's "Cult of Mercury" is not historical at all.
In the published setting, Bonisagus' theory is built on the "Cult of Mercury". Older traditions, on which this cult was based, were either not yet known by Bonisagus, or he did not acknowledge them in his work. It is only the Seekers, who later start to unearth some of these ancient secrets of magic.
Changing this in a campaign is possible - but requires quite some rebuild of the ArM5 Order.
Cheers
I've been trying to explain that Hermeticism is not regarded as a religion by its adherents, post Late-Antiquity at least. In my view, it's an occult philosophy embedded in the writings of Bonisgaus and others that provides a common understanding of the universe among Magi. It would certainly influence the religious beliefs of Magi, depending on how seriously an individual Magus took the Hermetic philosophy, but it would not necessarily drive one to abandon Christianity, Islam, or any other religion.
Nothing in particular has ever been published describing the Cult of Mercury, so there's no basis for insisting that they were not fully aware practitioners of Hermetic tradition. The alternative is that they followed a completely made up tradition, since we both agree there's no historical model for this Cult.
If we're speaking of canon, Bonisagus is known to have travelled widely in search of magical and occult texts and traditions. It would be surprising if his accumulation of wisdom did not include contact with the Hermetic Corpus.
I'm curious as to what you think would need to change. I don't see any issues offhand.
So we are both now over the OP's claim of Hermeticism as the Religion of Magi - and can discuss it in another context. As a "common understanding of the universe among Magi", it would have to influence e. g. Houses Bjornaer, Merinita, Ex Miscellanea and Tremere. How much influence could this be, then?
You got it: the Cult of Mercury is completely made up in ArM5. It has no historical basis. In history there never was a cult of magic around roman roads (F&F p.26f), or Latin magi communicating by correspondence to jointly cast magic rituals giving power to their group (ArM5 p.9). And the historical collegia of Roman priests (see de.wikipedia.org/wiki/R%C3%B6mis ... erschaften - no English wiki entry available) had nothing to do with the ArM5 Cult of Mercury.
By ArM5 canon (HoH:TS p.15) the Seekers discover The Legend of the Old Ones (in the way outlined already in ArM2 Houses of Hermes p. 30f). So Bonisagus cannot have communicated it already. In this context the question is secondary, whether he made contact with the Hermetika or not.
Do you propose, that Hermeticism is only the Magic Theory of Techniques and Forms, spells, items, familiars and labs? Then Hermeticism is completely irrelevant for ArM5 - and of course no change is required to introduce it. But even a generally accepted unifying philosophy beyond Magic Theory and the Code of Hermes does not fit with the Order as described in ArM5.
Cheers
There's also Fenecil's research into the past in 5e and his conclusions, as mentioned in HoH:TL, page 39.
It's not a concealment --- the point is that the "wild Germans" know the association too. By calling his magic "Hermetic" Bonisagus is saying it has its origins in an older, less imperialistic tradition than the Cult of Mercury. Bonisagus is saying that his magic goes back to the true source; the Cult of Mercury was a Roman corruption, he is deliberately distancing his magic from that.
I can't be "over" that since I never advocated it in the first place. For that matter, the OP appeared to be suggesting Hermeticism, not "claiming" it. You might wish to be more careful not to put words in the mouths of other people.
I do not see Hermeticism as a Religion of Magi. I am interested in how Magi relate to religion, particularly in terms of how their experiences with magic affect that. In this sense, I suspect Hermeticism would lead to decidedly non-Orthodox forms of belief, within the broad structures of Christianity and other religions. This follows the pattern of people in the real world who believed themselves to possess occult insights into the universe. It makes much more sense to me than that Magi would blindly follow the teachings of the Church, or that they would ignore religion and act like 21st century scientists.
The view that the Order of Hermes is itself the most out of place feature in Mythic Europe, now that 5e has elaborated historical and legendary detail beyond anything in previous editions, is widely held on here and I certainly share that view. A magical society with no common philosophical belief structure, no real or imagined occult insights, and no relationship between their magic and their religious belief feels not only anachronistic but artificial. In a time when mundane craft guilds professed mystical secrets, an Order which rejects such secrets and casts world altering magics based on completely empirical principles feels badly out of place.
Exploring the ideology and philosophy of Magi fits the pattern of gradually deepening the detail and richness of Mythic Europe. You certainly may disagree but I do not see any particular conflicts with the published history or structure of the Order.
That makes sense to me and is also consistent with my suggestion, reflecting the old 2e information, that the Cult of Mercury had even older sources and that Bonisagus was aware of those sources.
The older sources information is in 5e, too, as part of Fenecil's research.
Yes, although whether that is actually true or not, and if it is how important these older sources actually are, is a different set of questions. The truth of it doesn't need to be, and shouldn't be IMO, answered in the RAW canon --- but could be a question that PC magi seek to answer (if that sort of thing concerns them).
Let's get back to https://forum.atlas-games.com/t/on-the-religion-of-magi/9989/1:
to https://forum.atlas-games.com/t/on-the-religion-of-magi/9989/1:
and to https://forum.atlas-games.com/t/the-break-room/102/1:
That is more than "suggesting".
I am with you here. Mythic Europe holds many religions and faiths, which magi can adhere to. Only very specific magic traditions - like HoH:MC p.33ff The Huntress of the Woods, or TMRE p.124ff The Children of Hermes - require a specific faith from their followers as a matter of course.
People following Hermeticism, more structured philosophies like neoplatonism, or the budding aristotelianism of 13th century medieval academy, take an intellectual approach to faith, and thereby are very unlikely to follow any religion blindly.
For a Church father with such an intellectual approach look at Augustine (see en.wikipedia.org/wiki/Augustine_of_Hippo), who also in Mythic Europe inspires many medieval scholars.
Already the Gift and the existence of dozens of powerful individuals practicing magic are artificial features introducing 20th century fantasy concepts into the game world of ArM5.
The ArM5 Order of Hermes attempted to gather all the powerful, Gifted practitioners of magic in 8th century Mythic Europe - within the geographical limits of its reach. The purpose was to make a lasting peace among them, and to regulate all remaining violence. Religions or philosophies explicitly did not play any role in this: even Guorna the Fetid was invited. So the Order started as a very mixed bunch, and at uneasy peace.
Even the idea of a lasting peace among individuals disparate to such a degree, and not sharing a common religion, is completely anachronistic: the typical 13th century man would consider it a miracle, or a deceit of the tempter.
Magic Theory, however, is not just empirical: the roles of experience and reason in magic are contended between The Collegium and The Lyceum (see A&A p.10). As Hermetic magi have learned magic as apprentices of others, many who do not follow such discussions will practice magic as a highly complex art.
In Mythic Europe, those testing the limits of natural philosophy adopt the classical term empirici for themselves (see A&A p.67). You should carefully verify the time, when "mundane craft guilds" who "professed mystical secrets" appeared.
There can be many magi honestly believing that the Order of Hermes needs a common faith. In some campaigns they may even succeeed in proselytizing it.
But I see more magi with an intellectual approach to their own religion, who use classical and medieval philosophy to find and understand their role in the universe. And to such magi also Hermeticism can be useful.
Cheers
Honestly I could see a new mystery cult popping up every century with a new "unifying" religion they believe the entire order should adopt...