No one knows.
All right, that made an unsatisfying post. Let’s see if we can take that further.
There are at least three versions of the Grimoire of Honorius – the 1670, the 1760, and the 1800 – all of which are available in French on Joseph Peterson’s CD. As my French is only passable on a good day, I’ll be examining two different English texts. The first is The Grimoire of Pope Honorius issued by the Society of Esoteric Endeavour, which most closely mirrors the 1800 French text. The second is, unfortunately, from that Simon guy’s translation of the 1670 edition, so I guess we’ll just have to make do.
Now, the first book is attributed to Pope Honorius II and the other one to Pope Honorius III. Though around the seventeenth century the association by Protestants of Catholic practices with magic in order to discredit it was in full force, there was no particular reason to pick these individuals, neither of whom seemed particularly infamous. Nineteenth-century occultist Eliphas Levi attributed the book to the anti-pope Honorius II, which, as Waite points out in his Book of Ceremonial Magic, doesn’t make any sense either. Unfortunately, Waite then goes on to suggest a 1529 manual of exorcism called Honorii Papæ, adversus tenebrarum Principem et ejus Angelos Conjurationes ex originale Romæ servato was the source, without actually having seen it himself.
The most likely answer is that an enterprising author, drawing on the reputation of the influential but then rare Sworn Book, drew upon the legend of Honorius of Thebes. Either the author did not know that this Honorius was not a pope, or did not care. Certainly attributing the book to a famous figure like one of the Popes Honorius of old made for a more effective sales pitch. Of course, this is a hypothetical scenario, but it seems to be the most likely one.
Next time, the origins (yes, more than one) of the Grimoire of Honorius.