Indeed, Justin tells us the way the gospels (“memoirs of the apostles”) were valued in his day: “And on the day called Sunday, all who live in cities or in the country gather together to one place, and the memoirs of the apostles or the writings of the prophets are read, as long as time permits; then, when the reader has ceased, the president verbally instructs, and exhorts to the imitation of these good things” (1 Apol 67.3). Such a worship practice was not invented by Justin, but seems to be a practice with a lengthy historical pedigree. Thus, there are good reasons to think that the origins of the fourfold gospel probably go back even further.
There has been a long-standing scholarly discussion about how far back we can trace the roots of the fourfold gospel. Even though the four gospels obviously existed in the first century, there is still debate about when Christians began to gather them together and view them as a unit.
We certainly see this happening in Irenaeus, who is quite plain about his view, “It is not possible that the gospels can be either more or fewer than the number they are. For, since there are four zones of the world in which we live and four principle winds… [and] the cherubim, too, were four-faced” (Haer. 3.11.8).
But, can we trace the fourfold gospel back even further? Justin Martyr, an early Christian apologist writing c.150-160, is a key player in this debate. He clearly knows the Synoptic Gospels—Matthew, Mark and Luke. But did he know John? Scholars disagree about this. But, I think there are good reasons to think that he did. Here are a few:
1. We should remember that Justin was the teacher and mentor of Tatian who was famous for producing a harmony of all four gospels known as the Diatesseron. It is noteworthy not only that John was included in Tatian’s harmony, but that John provided the central chronological backbone for his work. If Tatian valued John so highly, then it is difficult to believe that his mentor, Justin, would have been unaware of this gospel.