The Gospels and the Gospel
In all of Justin’s extant writings, the word “gospels” appears only once, in his First Apology, while the word “gospel” is used twice in his book Dialogue with Trypho the Jew. Concerning the references to “the Gospel” in Trypho, which some have taken for knowledge of the canonical gospels, the Church father evidently is referring to another text altogether.
Walter Richard Cassels states, in
Supernatural Religion, (which includes an 85-page, detailed study of Justin’s work vis-à-vis the canonical gospels, with the original Greek and Latin, along with copious notes and citations), -
The title, 'Memoirs of the Apostles', by no means indicates a plurality of Gospels. A single passage has been pointed out in which the Memoirs are said to have been called εὐαγγέλια in the plural: “For the Apostles in the Memoirs composed by them, which are called Gospels,” etc. The last expression, ἁ καλειται εὐαγγέλια [“which are called Gospels”], as many scholars have declared, is probably an interpolation. It is, in all likelihood, a gloss on the margin of some old MS. [manuscript] which copyists afterwards inserted in the text. If Justin really stated that the Memoirs were called Gospels, it seems incomprehensible that he should never [elsewhere] call them so himself. In no other place in his writings does he apply the plural to them, but, on the contrary, we find Trypho [in Justin’s Dialogue with Trypho] referring to the “so-called Gospel,” which he states that he has carefully read, and which, of course, can only be Justin’s “Memoirs”; and, again, in another part of the same dialogue, Justin quotes passages which are written “in the Gospel” (εν τω ευαγγελίω γέγραπται). The term “Gospel” is nowhere else used by Justin in reference to a written record. In no case, however, considering the numerous Gospels then in circulation, and the fact that many of these, different from the canonical Gospels, are known to have been exclusively used by distinguished contemporaries of Justin, and by various communities of Christians in that day, could such an expression be taken as a special indication of the canonical Gospels.
“The one instance of ‘gospels’ in Justin appears to be a scribal marginal gloss and explanatory note that was interpolated into the text.”
... the one instance of “gospels” in Justin appears to be a scribal marginal gloss and explanatory note that was interpolated into the text. Otherwise, it would be impossible to explain why Justin only uses this word once in all of his writings. Hence, the term’s appearance in his book is not an identification by Justin himself of the Memoirs with the gospels. The other “gospel” usages in Justin concern a single text commonly understood in his circle as “the Gospel,” possibly the text Justin calls the 'Memoirs of the Apostles'.
http://freethoughtnation.com/does-justin-martyr-quote-the-gospels/
If Justin actually had the canonical gospels before him when writing his texts, he could only be considered sloppy in his citations, which is the accusation made to explain why his “Memoirs” differs so much from the gospels. The reality is that [this] Church father is surprisingly consistent and conscientious in his quotation elsewhere. For example, as I state in SOG, Martyr quotes from the Old Testament 314 instances, 197 of which he names the particular book or author, equaling an impressive two-thirds of the total amount. Several of the other 117 instances may not have needed citation, “considering the nature of the passage.” Despite his remarkably fastidious record, when Justin is supposedly quoting the New Testament, he mentions none of the four gospels. Instead, he distinctly states that the quotes are from the “Memoirs” ...
The facts are that the terms “gospels” and “gospel” in Justin do not indicate his knowledge of our canonical gospels; that the quotes from the Memoirs of the Apostles are not the same as those in the canonical gospels; and that the term “Memoirs” appears to refer to a single text, like “Acts of the Apostles,” rather than serving as a reference to the gospels of Matthew, Mark, Luke and John, whom Justin does not mention or even seem to know. In the final analysis, it is evident that Justin Martyr does not quote the canonical gospels and that, despite the wishful thinking, these texts do not emerge clearly in the historical record until the end of the second century.