I'm going to bump this to add some evidence to the whole issue, as I was revisiting this and wanted to dig deeply. Parker referenced that this letter of Pope St. Dionysius of Alexandria to Pope St. Sixtus came from, what was then at the turn of the century, "newly discovered manuscripts" at the British Museum labeled Nos. 12151-2
Jerome informs us (Scr. Ecc. 46) that Pantaenus6, one of the most celebrated Christian philosophers of Alexandria, was sent, A.D. 193, by Demetrius, Bishop of that city, to India, at the request of a delegation from India for that purpose. Pantaenus discovered, on his arrival, that St. Bartholomew (one of the twelve) had preached the coming of Jesus Christ, in that country. Pantaenus found a copy of the Hebrew Gospel of St. Matthew in India. Now, by the extract, contained in the Scholia of Maximus, from the Scholia of Dionysius of Alexandria (250) upon the Divine Names, and also by the extract from a letter of the same Dionysius, recently discovered in the British Museum7 (Nos. 12151-2), we know that the writings of Dionysius the Areopagite were known and treasured in Alexandria a few years after the death of Pantaenus. Can we reasonably doubt that Pantaenus took the writings of Dionysius, and the more abstract works of Hierotheus, to India? Have we not here an explanation of the remarkable similarity between the Hindu philosophy, as expressed by Sankara8 in the eighth, and Râmânuja in the thirteenth century, and the "Divine Names?" Sankara treats of the Supreme as "absolutely One;" Râmânuja as "non-dual, with qualification." Both these truths are combined and expressed in Dionysius.
6 Conversion of India, p. 12. Pressensé, The Earlier Years of Christianity, Vol. II. p. 271. The History of Mathurâ (Muttra), by F. S. Growse, on the glorification of the Divine Name.
7 Vidieu, p. 73.
8 Sankara’s doctrine, Sir Monier Williams, "Brahmanism," p. 55. Râmânuja’s explained, "Brahmanism," p. 119, &c. J. Murray.
I highlighted the reference "Vidieu" and after doing some searching, found the French work, and will provide the link here:http://books.google.com/books?id=TGhHAAAAYAAJ&ots=kUF0fzuaPF&dq=Vidieu%20dionysius&lr&pg=PA73#v=onepage&q=Il%20nous%20reste%20d%20ailleurs%20de%20saint%20Denys%20d%20Alexandrie%20une%20lettre%20au%20pape%20&f=false
This should directly take you to p. 72 of the work. Until someone provides a better French translation, I'm going to provide the Google Translate function as best as I could utilize it:
It remains also of St. Dionysius of Alexandria a letter to Pope St. Sixtus II, which, if genuine,1 is an argument in favor of our thesis. It was found quite recently by Father Martin in two codices of the British Museum.2 In the first, we read fragments of this letter at the end of the apology of John Scholasticus(605) under the title:
"New apology made by George, priest of the great church of Constantinople and from the town of Baïchan(Bosra), about the divine writings that are repudiated by ignorant, as if they were not the work of this great doctor (Dionysius), but only the production of some heretic, Apollinaire, for example, or any new and unknown heretic. "
But this writer, George, priest of Constantinople, after recounting how the books of the Areopagite had been repeatedly rejected by foolish men, says he brings an argument such that it will shut the mouths of any opponent, and this argument is the letter of Dionysius of Alexandria, which reads in part:
(please refer to the OP for the quoted text)
1 There is no evidence that it is not, we have the right to be regarded as authentic until proven otherwise.
2 Nos 12151, 12152
If anyone who reads French has more to add in the context of the next few pages before and after the quoted area, I would greatly appreciate a summary or translation if you feel it is necessary and helpful for this discussion. At the time, it seems that they considered the said letter to be authentic, even though there seemed to be a debate that it was not, some thinking that it was an Apollinarian forgery (where did we hear that before
The "John Scholasticas" is most probably John of Scythopolis, who seemed to be heavily invested at the time in trying to prove the authenticity of the Dionysian corpus. When I did a google search, of him along with the words "dionysius alexandria sixtus", I was lead to the scholarly book by Rorem on page 105:http://books.google.com/books?id=Pr3H5CefVh4C&pg=PA105&lpg=PA105&dq=john+of+scythopolis+dionysius+to+sixtus&source=bl&ots=PrZNmfE43l&sig=Mj7Py8XWs9tcWyTlTbZWYka9Utk&hl=en&sa=X&ei=szgyU4X5A8bo0QH79YCYDQ&ved=0CCYQ6AEwAA#v=onepage&q=john%20of%20scythopolis%20dionysius%20to%20sixtus&f=false
Rorem here puts a footnote explaining the alleged letter by the Alexandrian pope is "clearly a forgery".
All in all, this has been interesting, and while I'm not sure if Rorem refutes good 'ol Parker's defense back in the late 1890s, it does lead somewhere in this nice little case study.