The Diatessaon is an attempt at a Harmony of the four canonical Gospels. It was complied by Titian in about AD172, but it contains readings whihc imply that he used versions of the canonical gospels some of which contain pre markan elements.
In an article published in the Back of Helmut Koester's Ancient Christian Gospels, William L. Petersen states:
"Sometimes we stumble across readings which are arguably earlier than the present canonical text. One is Matthew 8:4 (and Parallels) where the canonical text runs "go show yourself to the priests and offer the gift which Moses commanded as a testimony to them" No fewer than 6 Diatessaronic witnesses...give the following (with minor variants) "Go show yourself to the priests and fulfill the law." With eastern and western support and no other known sources from which these Diatessaranic witnesses might have acquired the reading we must conclude that it is the reading of Tatian...The Diatessaronic reading is certainly more congielian to Judaic Christianity than than to the group which latter came to dominate the church and which edited its texts, Gentile Christians. We must hold open the possibility that the present canonical reading might be a revision of an earlier, stricter , more explicit and more Judeo-Christian text, here preserved only in the Diatessaron.
The Jesus Narrative In Pauline Literature
Paul's allusions to the narrative relates to many points in the Gospels:
|He was flesh and blood||(Phil 2:6, 1 Tim 3:16)|
|Born from the lineage of David||(Rom 1:3-4, 2 Tim 2:8)|
|Jesus' baptism is implied||(Rom 10:9)|
|The last supper||(1 Cor 11:23ff)|
|Confessed his Messiahship before Pilate||(1 Tim 6:13)|
|Died for peoples' sins||(Rom 4:25, 1 Tim 2:6)|
|He was killed||(1 Cor 15:3, Phil 2:8)|
|Buried||(1 Cor 15:4)|
|Empty tomb is implied||(1 Cor 15:4)|
|Jesus was raised from the dead||(2 Tim 2:8)|
|Resurrected Jesus appeared to people||(1 Cor 15:4ff)|
|James, a former skeptics, witnessed this||(1 Cor 15:7)|
|as did Paul||(1 Cor 15:8-9)|
|This was reported at an early date||(1 Cor 15:4-8)|
|He asceded to heaven, glorified and exalted||(1 Tim 3:16, Phil 2:6f)|
|Disciples were transformed by this||(1 Tim 3:16)|
|Disciples made the Gospel center of preaching||(1 Cor 15:1-4)|
|Resurrection was chief validation of message||(Rom 1:3-4, Rom 10:9-10)|
|Called Son of God||(Rom 1:3-4)|
|Called Lord||(Rom 1:4, Rom 10:9, Phil 2:11)|
|Called God||(Phil 2:6)|
|Called Christ or Messiah||(Rom 1:4, Phil 2:11|
|Summary and Conclusion|
Koster and Crosson both agree that the PMR was circulating in written form with empty tomb and passion narrative, as early as 50AD
From this notion as a base line for the begining of the process of redaction, and using the traditional dates given the final product of canonical gospels as the base line for the end of the process, we can see that it is quite probable that the canonical gospels were formed between 50 and 95 AD. It appears most likely that the early phase, from the events themselves that form the Gospel, to the circulation of a written narrative, there was a controlled oral tradition that had its hay day in the 30's-40's but probably overlapped into the 60's or 70's. The say sources began to be produced, probably in the 40's, as the first written attempt to remember Jesus' teachings. The production of a written narrative in 50, or there abouts, probably sparked interest among the communities of the faithful in producing their own narrative accounts; after all, they too had eye witnesses.
Between 50-70's those who gravitated toward Gnosticism began emphasizing those saying sources and narrative pericopes that interested them for their seeming Gonostic elements, while the Orthodox honed their own orthodox sources that are reflected in Paul's choices of material,and latter in the canonical gospels themselves. So a great "drying up" process began where by what would become Gnostic lore got it's start, and for that reason was weeded out of the orthodox pile of sayings and doings. By that I mean sayings Like "if you are near to the fire you are near to life" (Gospel of the Savior) or "cleave the stone and I am there" (Thomas) "If Heaven is in the could the birds of the air will get there before you" (Thomas) have a seeming gnostic flavor but could be construed as orthodox. These were used by the Gnsotically inclined and left by the orthodox. That makes sense as we see the earliest battles with gnosticism beginning to heat up in the Pauline literature.
My own theory is that Mark was produced in several forms between 60-70, before finally coming to rest in the form we know it today in 70. During that time Matthew and Luke each copied from different versions of it. John bears some commonality with Mark, according to Koester, becasue both draw upon the PMR. Thus the early formation of John began in 50-s or 60s, the great schism of the group probably happened in the 70's or 80s, with the gnostic bunching leaving for Egypt and producing their own Gnostic redaction of the gospel of John, the orthodox group then producing the final form by adding the pro-luge which in effect, is the ultimate censor to those who left the group.
The Gospel material was circularizing throughout Church hsitory, form the infancy of the Church to the final production of Canonical Gospels. Thus the skeptical retort that "they weren't written until decades latter" is totally irrelevant. It is not the case,they were being written all along, and they were the focus of the communities from which they sprang, the communities which originally witnessed the events and the ministry of Jesus Christ.
 William L. Petersen Titian's Diatessaron in Helmut Koester, Ancient Christian Gospels: Their History and Development, Philadelphia: Trinity Press International, 1990, p. 424