One stray ante-Nicene work that never appeared in the Ante-Nicene Fathers collection is a dialogue On the true faith in God attributed in the manuscripts to an otherwise unknown Adamantius. Of course Origen was known as Adamantius also, but the author of this work holds anti-Origenist views.
The work consists of a dialogue in two parts, held with the pagan Eutropius as arbiter, on which is the real Christianity. In the first part, the author disputes with two Marcionites. In the second a follower of Bardesanes is refuted.
The text makes use of now lost works by Methodius, and therefore cannot be much earlier than 300 AD. It is extant in the original Greek in at least ten manuscripts, and also in a translation by Rufinus. It was published in the Patrologia Graeca 11, and a critical text exists in the GCS vol. 4 (1901) which is online, albeit only to Americans and contains both the Greek and Rufinus’ Latin. An English translation by Robert A. Pretty was published in 1997 by Peeters, but sadly is offline.
I have been sent a quotation from the work, or rather what is apparently a paraphrase of a passage from it, which is as follows:
“What right has he [a heretic] to assert that the Messiah wrote the gospel? The gospel writer did not refer to himself as the Christ but to Jesus who he is proclaiming.”
I have no idea where in the dialogue this can be found. Does anyone have any ideas?