Tatian's Diatessaron, it turns out, is a late second century harmony of the Gospels. The text takes much of its framework from the Fourth Gospel and incorporates much of the synoptic material into a single Gospel.* It is a really fascinating window into Christian "social" memory. In this case, I would call it Christian "counter memory". In my view, the Diatessaron was an attempt to replace the fourfold Gospel tradition. After all, wouldn't one Gospel be better than four? Just think of how many biblical scholars** would have been diverted to other topics if we only had a single narrative, chronology, sayings corpus, etc. Tatian is generally thought to be interested in "smoothing out the differences" between the Gospels. Of course, this gets awkward when then the story of Nathanel is recounted (cf. John 1:45ff) followed three chapters later by Luke's list of 12 disciples—suggesting 13 disciples. But of course, Tatian consistently refers to the twelve disciples throughout. Just an example to tempt your tummy.
Just think of all the implications for social memory, Bible nerds! Certain memories are omitted (thus mnemonic distanciation); certain memories are conflated (thus mnemonic cross-pollination***); all the while in the context of overt mnemonic narrativization. But what makes the Diatessaron an absolute hoot is the fact that it provides a window into how particular formulations of commemoration were rejected by the collective (and I mean this in the broadest sense possible). I must underscore my point here: the Diatessaron has been well researched; I'm not suggesting that I just found it in a monastery yesterday. What makes this topic ripe for further research is the social memory angle. It's a trendy topic, but one with a good four-score left in the tank.
*NB: G.F. Moore's estimate was that it incorporates 50 percent of Mark, 66 percent of Luke, 76.5 percent of Matthew, and 96 percent of John - I wouldn't place too much weight on these numbers, but it gives you a general picture.
**NB: By "scholars", of course, I mean heretics.
***NB: © Anthony Le Donne.