A superficial reading is enough to see that there is an underlining assumption made by Koester, that Thomas preserves primitive Jesus sayings when compared to the canonical gospels. How should one take this claim? Is Thomas an independent, reliable, extra-canonical source, for Jesus' sayings?
"If one considers the form and wording of the individual sayings in comparison with the form in which they are preserved in the New Testament, The Gospel of Thomas almost
The Nag Hammadi Codices
always appears to preserve a more original form of the traditional saying (in a few instances, where this is not the case, the Coptic translation seems to have been influenced by the translator's knowledge of the New Testament gospels), or presents versions which are independently based on more original forms." (Robinson, 125)
In his recent work Thomas and the Gospels: The Case for Thomas’s Familiarity with the Synoptics, Mark Goodacre argued (as the title suggests) that the author of Thomas was familiar with the redacted, completed form of the synoptic gospels, see especially his chapter discussing saying 79 (Goodacre, 97-108). Goodacre spends considerable time working with the early Greek fragments of Thomas found at Oxyrhynchus as they predate the Coptic version found at Nag Hammadi by one hundred years or more and (most likely) represent an earlier form of the document. These are P. Oxy. 1 ca. 200 CE, P. Oxy. 655 ca. 200-250 CE, and P. Oxy. 654 ca. 250 CE (Ricchuiti, 196).
Goodacre points to several lines of evidence in order to draw attention to Thomas’s familiarity with the synoptics, but this post will highlight only two features in particular. First, there are several word-for-word agreements between the synoptics at several places suggesting that the author(s) of Thomas referenced the completed text of the gospels while writing (see especially saying 26, Goodacre, 30-33). And second, the author(s) of Thomas left out important elements of Jesus’ pericopes which indicates the author(s) had an intimate familiarity with the sayings as they appear in the synoptics (Goodacre, 109-127). For example, because everyone is familiar with the saying “When in Rome, do as the Romans do,” most anyone can simply say “When in Rome . . . “ with out finishing the sentence because most anyone will immediately recognize the saying and be able to finish it in their minds. The same can be said concerning the author(s) of Thomas and Jesus' sayings. They were so familiar to the author(s) that they attempted to make the saying more cryptic but failed to include important details that were essential to the story and plot of the pericope.
A second major clue that testifies against the credibility of Thomas as a source concerns the transmission of its vorlage. Timothy Ricchuiti compared the earlier Greek fragments of Thomas with the much later (by one hundred years at least) Coptic translation of the gospel. He noted that at several places there were significant variations between them. Note especially a comparison of P. Oxy. 655 at logion 24, which reveals a very large section of the Greek text that has been edited out by the redactors of the Coptic Thomas for most likely theological reasons (Ricchuiti, 221-223). There are several other places that could be mentioned that involve theological redaction in the Coptic text when compared with the earlier Greek fragments, such as logion 2,3,5, and 6 (Ricchuiti, 210). Overall, Ricchuiti’s study concluded that “[i]t does indeed appear that the Coptic scribe altered Thomas in such a way as to make it more amenable to the community that eventually decided to include it in the Nag Hammadi writings” (Riccuiti, 228).
The Secret Jesus
Many scholars are eager to search Thomas for primitive material in reconstructing the historical Jesus because there are so many familiar sayings of Jesus intertwined throughout the work (Robinson, 124-125). It appears that the author(s) of Thomas were keen to present sayings of Jesus that were familiar to the readers of the canonical gospels. But why?
Mark Goodacre suggested that the most likely reason the author of Thomas used the synoptics in composing Thomas was to "authenticate" the "newer, stranger material" (Goodacre, 172). He continued:
"It is no accident, in other words, that Thomas interlaces Synoptic and non-Synoptic material, two or three sayings at a time, always keeping the sound of the Synoptic Jesus close at hand while interweaving sayings from Thomas's enigmatic, secret Jesus." (Goodacre, 172)Therefore, it appears that the author(s) of Thomas were dependent on the synoptics in order to theologically alter the appearance of Jesus as an enigmatic Gnostic figure. Couple this with the apparent theologically motivated scribal alterations from the Greek to the Coptic version, and one must come to the conclusion that Thomas is not a reliable source for authentic sayings of Jesus.
Echoes of the Historical Jesus
Despite the apparent theological tampering by the author(s) of Thomas, there are clues that suggest Thomas contains echoes of extra-canonical Jesus sayings. One intriguing clue concerns saying 22 which is quoted (albeit loosely) in 2 Clement 12.2, a second century sermon, as well as several other early Christian sources (Holmes, 102). Because there is wide and early attestation to logion 22, it is very likely that there are authentic echoes of Jesus contained in this saying.
Therefore it is not wise for scholars to dismiss Thomas out of hand, but rather, Thomas should be approached as any other ancient document, on its own terms as the author(s) originally intended the work to be read (Goodacre, 174).
 For an excellent summary of the ancient sources concerning this logion see http://www.earlychristianwritings.com/thomas/gospelthomas22.html
Goodacre, Mark. Thomas and the Gospels: The Case for Thomas’s Familiarity with the Synoptics. Grand Rapids: Eerdmans, 2012.
Holmes, Michael W. ed. The Apostolic Fathers: Greek Texts and English Translations. Updated Edition. Grand Rapids: Baker, 1999.
Ricchuiti, Tim. “Tracking Thomas: A Text-Critical Look at the Transmission of the Gopel of Thomas.” Pages 189-228 in Revisiting the Corruption of the New Testament: Manuscript, Patristic, and Apocryphal Evidence. ed. Daniel B. Wallace. Grand Rapids: Kregel, 2011.
Robinson, James M., and Richard Smith. The Nag Hammadi Library in English. Leiden: E.J. Brill, 1988.
This post was adapted from a response I made to a forum discussion question in one of my classes.