Some folks (most Muslim, but now one Roman Catholic) would like to imagine that Mohamed was not addressing Christianity, but rather some sect that held to a Trinity of Allah, Jesus, and Mary, which we refer to (for convenience) as FSM-trinitarianism.
Leaving aside the internal evidence of Surah 5 (which defeats that argument), there is a dearth of external evidence to substantiate the existence of such a sect.
David Waltz, the Roman Catholic mentioned above, quoted the following:
"The three gods in the Koran (c. 4, p. 81, c. 5, p. 92) are obviously directed against our Catholic mystery: but the Arabic commentators understand them of the Father, the Son, and the Virgin Mary, an heretical Trinity, maintained, as it is said, by some Barbarians at the Council of Nice, (Eutych. Annal. tom. i. p. 440.) But the existence of the Marianites is denied by the candid Beausobre, (Hist. de Manicheisme, tom. i. p. 532;) and he derives the mistake from the word Roxah, the Holy Ghost, which in some Oriental tongues is of the feminine gender, and is figuratively styled the mother of Christ in the Gospel of the Nazarenes." (Gibbon, The Decline and Fall of the Roman Empire, ch. 50, The Modern Library edition (n.d.), pp. 81, 82.)
Before we get to the rest of the analysis, it's important to note:
1. That Gibbon, the historian providing the discussion, confirms the opinion set forth in my previous post, namely that Mohamed is attempting to interact with the doctrine of the Trinity. In fact, he goes so far as to say that it is obvious.
2. Gibbon notes that there is a dissent by the "Arabic commentators," and identifies their source, but does not endorse that dissent.
3. Gibbon, in fact, cites evidence that undermines the dissent, and provides an explanation as to how Mohamed may have been confused by feminine reference to the Holy Ghost.
Incidentally, one can read the matter in context here: (link).
Until I read the above, I had never heard of attendees of the Councial of Nicea maintaining of a view of the Trinity that consisted of “the Father, the Son, and the Virgin Mary”. (any errors in original)
That's not surprising of course. Gibbon mentions such evidence, but dismisses it. It's interesting to note what David seems to observe and what he seems to overlook. Nevertheless, while we could simply dismiss the matter with Gibbon, let's dig deeper into the matter.
"Gibbon’s reference is to Eutychius’ (10th century Patriarch of Alexandria, and noted historian) Annals. This work is found in volume 111 of Migne’s Patrologiæ Græca, a volume, which until this morning, I did not have access to. I wanted to check Migne’s before mentioning Gibbon. " (any errors in original)
Eutychius was a 10th century Melkite Patriarch in Alexandria. He reigned in the Coptic church in Egypt during a time when he was surrounded by the ruling majority of Fatamid Muslims. To maintain the position I presented on the previous page would have been dangerous to his health. As an interesting aside, he was also a Young Earth Creationist (placing Creation 5,500 years years before Christ).
There are several salient facts:
1) Eutychius lived more than 500 years after the council of Nicea.
2) Eutychius did not cite any evidence to support his claim, or explain in any way how he obtained this supposed information regarding Nicean attendees.
3) Eutychius' claim was only that there were some who showed up at the Nicean council with such views. Eutychius, for example, did not even claim that they left with those views. Furthermore, Eutychius does not suggest that these folks left spiritual descendants that survived for centuries until the time of Mohamed.
4) Eutychius is criticized for his weakness as an historian, by historians such as E. W. Kemp and C. Wilfrid Griggs (adopting Kemp's position) (link). Likewise David Cook indicated (and Gary W. Kronk adopted) that the Annales are "riddled with errors." (link) See also, for similar comments (Philip Rousseau) (William Smith et al.) (William Scott et al.) (cf. response of Eutychian supporter)
Dave continued by providing a quotation:
Here is the quote from Migne’s: Mittens ergo Constantius rex in omnes passim regiones, patriarchas et episcopos convocavit, adeo ut post annum et duos menses Niceæ convenirent his mille quadraginta octo episcopi, sententiis et religionibus inter se discrepantes. Erant ex illia qui affirmarent Christum et Matrem ipsius duos esse deus præter Deuni [summon :] errant hi Barbari, et Marianitæ audierunt. (Patrologiæ Græca, Tomus CXI, col. 1005, sec. 439-440.) (errors in Waltz's transcription are his own)
The correct quotation in key part is actually, "Erant ex illia qui affirmarent Christum et Matrem ipsius duos esse deos praeter Deum [summum:] erant hi Barbari, et Marianitae audierunt." which being roughly translated is: "There were among them those that affirmed Christ and his Mother to be two gods besides God [the most high:] there were here Barbarians and Marianites heard."
Furthermore, of course, this is only the Latin translation of what was presumably an Arabic original.
The full page can be read here (right hand column, clustered around the 440). A partial reproduction is shown below, click to expand the image for easier reading.
David concluded: "So, as you can now see for yourself, we now have Christian source for the existence of at least two Christian groups/sects that held to FSM (God [the Father], Christ, and Mother "in one"), backing up the testimony from the Qur'an."
a) Leaving aside the question of the interaction between the Melkites and the Copts (on the one hand) and the Muslims (on the other hand), the fact that Eutychius was not Muslim is not very strong evidence against bias.
b) Eutychius suggests that the group came to Nicea, but does not call them Christians: in fact, he calls them Barbarians and Marianites.
c) Eutychius' comment "Barbarians and Marianites" should probably be understood as heaped insults, rather than as a listing of two separate groups that held the same thing.
This is the best evidence that has been presented to try to rescue Mohamed's claim about FSM Trinitarians, but it is more or less strawy. It's a single witness who comes half a millenium after the alleged event took place. Furthermore, it's an event that predates Mohamed's claim by two centuries. Moreover, it is an event that is not described as having any geographical proximity to Mecca/Medina or even Arabia. It is an event alleged by a man living among Muslims. It is an event for which the man does not cite his sources. It is an event that is included in a book that has been repeatedly criticized for historical mistakes. It is an event that has (so far) only been conveyed to us in a Latin translation of an Arabic original.
In short, we can easily overcome the best evidence to support the existence of FSM-trinitarians, which was presented in order to provide an out for Mohamed under a hypothesis that Mohamed was (in some parts of Surah 5) addressing an heretical sect that has since disappeared. In short, the previously presented theory that Mohamed simply did not understand the doctrine of the Trinity has survived the challenge, and we may continue to safely assume that there were no FSM-trinitarians running around at the time Mohamed lived.
UPDATE: corrected Sunni to Fatamid, above, thanks to David Waltz.