To my eyes, the biggest clue to the historical answer seems to lie in the large number of references to a mysterious ‘Book M’ scattered through the Fama and the Confessio.
The Fama says that Book M is written for all time:
[…] he did bind those [first] three [of his Brethren] unto himself to be faithful, diligent, and secret; as slo to commit carefully to writing all that he should direct and instruct them in, to the end that those which were to come, and through especial Revelation should be received into this Fraternity, might not be deceived of the least sillable and word.
This Book M is written in a magical language, and contains the secrets of the world:
After this manner began the Fraternity of the Rosy Cross; first, by four persons onely, and by them was made the Magical Language and writing, with a large Dictionary, which we yet dayly use to Gods praise and glory, and do finde great wisdom therein; they made also the first part of the Book M: but in respect that the labour was too heavy, and the unspeakable concourse of the sick hindred them, and also whilst his new building (called Sancti spiritus) was now finished, they concluded to draw and receive yet others more into their Fraternity […], eight in number, all batchelors and of vowed virginity, by those was collected a book or volumn of all that which man can desire, wish, or hope for.
The magical language is something that nobody outside of the Rosy Cross can read:
[…] notwithstanding our hand-writing which is in our hands, no man (except God alone) can make it common, nor any unworthy person is able to bereave us of it.
Book M contains many chapters and drawings related to Chymia (chemistry):
Also we do testifie that under the name of Chymia many books and pictures are set forth in Contumeliam gloriae Dei, as we wil name them in their due season, and wil give to the pure-hearted a Catalogue, or Register of them:
The Confessio says that Book M contains all the useful knowledge contained in all other books:
Were it not a precious thing, that you could so read in one onely book, and withal by reading understand, and remember all that which in all other books […] hath been, is, and shal be learned, and found out of them?
Because of this, its contents are secret and will never be made widely known:
But withal we signifie so much, that our Arcana or Secrets will no ways be common, and generally made known.
The Rosicrucians’ adeptness at using their magical language to express the true nature of all things has meant that they are less adroit at using other languages:
From the which Characters or Letters we have borrowed our Magick writing, and have found out, and made a new Language for ourselves, in the which withal is expressed and declared the Nature of all Things. So that it is no wonder that we are not so eloquent in other Languages […].
The Rosicrucians have access to medicine which will cure all diseases (though not plagues sent directly by God):
But let them think, that although there be a medicine to be had which might fully cure all Diseases, nevertheless those whom God hath destined to plague with diseases, and to keep under the Rod of Correction, such shall never obtain any such Medicine.
By way of contrast, while the Assertio does mention that the Rosy Cross has a book of knowledge that it accumulates, it is a far less elaborate and complicated affair, and there is no mention at all of anything like a magical language:
Then, everyone tells what he has seen, read, considered and heard; every one in his turn writes it then later, accurately, in a book assigned for that purpose, so that posterity will take cognizance of it.
My inference from all this is that the Book M containing drawings of Chymia and written in an impenetrable magical language was probably a feature introduced close to or in the Fama’s final writing phase. But why stitch this throughout the Fama? Why was it necessary for this book to sit at the heart of the Rosy Cross’s mythology?
The Secret History of the Rosicrucians (c) 2012, 2015 Nick Pelling.
2. The Three Texts
3. Dating The Fama And The Confessio
4. The Fama’s First Draft
5. So… What Was The Point Of It All?
6. ‘Book M’
7. Another Mysterious Manuscript
8. Stories From The Margins
9. Andreae’s Two Journeys
10. The Limits Of Evidence