I will get back to blogging the various patristic traditions about Mark soon, but in the meantime here is Mike Bird’s view on the emergence of the four Gospel canon. My own view is that the Apostolic Fathers tend to quote either independent oral tradition or, if they reference a written text (e.g., the Didache), it is most likely Matthew (see the studies by Koester, Massaux, Gregory and Tuckett, etc). The Synoptic Gospels and possibly John (and other sources?) are included in Justin Martyr’s “Memoirs of the Apostles,” memoirs written by the apostles or their assistants (Dial. 103.8). I think a fourfold gospel canon probably emerged sometime between Justin and Irenaeus; Irenaeus’ curious argument about why there are four and only four Gospels shows that he is defending an already existing fourfold Gospel canon that was popular but may have not yet won the day. Other Christian groups selected just one Gospel like the Marcionites (some version akin to Luke), the Ebionites (a Hebrew version of Matthew) or the Diatessaron or were willing to use both the New Testament Gospels and their own Gospels such as the Valentinians. What do you think?