Nevertheless, I think he does miss some important facets of that conversation, and these need some illumination. Horsley begins his concluding thoughts in this way:
As we have seen, recent research is undermining some of the assumptions on which standard study of the Gospels previously depended. We can recognize now that there were no stable written texts of Mark, Matthew, Luke, or John in late antiquity. the written texts were "unstable," that is, they kept developing, in part because any texts in antiquity, whether written down or not, were generally recited or "performed" before groups of people. Oral communication was predominant, even in literate circles. (Horsley 2008: 224)
Whatever we might think about the gospels, one of the things about which we can be most confident is that they were, indeed, not stable in the sense in which we think of stability. I am sitting in my office surrounded by bookshelves lined with printed texts, every single one of which has a copyright notice even before the title page and has been printed through a highly developed process that guarantees that every copy of that book is identical, even to the point of reproducing any errata. What's more, when I refer to these texts, I am careful to do so precisely and exactly, in part because I know that any one can check my references in their own copies of those texts.
The gospels did not have any of these features. Of the thousands of copies that survive as manuscripts, none of them are identical. While most of the differences are relatively insignificant, some are rather important. How/Where does Mark's gospel end? Where does the story of the woman caught in adultery belong in the canon, if anywhere? Similar questions attend every book of the New Testament. Does Paul, for example, tell his readers that they have peace with God [ἔχομεν; echomen], or does he exhort them to pursue peace with God [ἔχωμεν; echōmen; Rom 5.1]? This instability doesn't just attend the transmission of the written text but also the performance of the traditions contained therein. In other words, just because the written text of Matt 5.3–12 has a very specific number and order of beatitudes doesn't mean that every time the Matthean evangelist performed Jesus' covenantal sermon he began with that specific number/order of beatitudes. Obviously, when Luke performed a similar tradition, he had a different number of beatitudes, but I would also suggest that the Lukan evangelist also may have performed that tradition differently before as well as after writing the text we call Luke.
But I balk somewhat at calling this instability development because of the evolutionary trajectories that have structured our thinking about gospel traditions at least since the advent of form criticism. Whether or not Horsley intends "developing" in this sense, he is bound to be read this way among biblical scholars. I suggest that the texts' movement—French scholar Paul Zumthor uses the term mouvance in a highly technical sense—was a feature of its function rather than its development. In other words, later texts (Matthew and Luke?) were not "more developed" than earlier ones (Mark? Q??) even despite the differences between them. Instead, in very many instances, the differences between them manifest the tradition's transcendence of written texts. The Jesus tradition was evoked by the written texts rather than contained by them, and shifts in the text were shifts in respect to the larger Jesus tradition rather than shifts in the tradition itself.
I should say that the tradition itself could shift, could develop, and we may see that in the relation between the synoptic gospels and John, and again with the Gospel of Thomas (and even within Gos. Thom., if we follow April DeConick), and certainly in some of the other texts that call themselves gospels. But we don't detect that development in the tradition as a whole by tracing minute changes in wording. Despite the sometimes awesome changes a single word can manifest (e.g., "Spirit of God" versus "finger of God" in Matt 12.28||Luke 11.20), the tradition often remains intact even as the evocation of the tradition responds to the dynamic and vibrant life of the communities cherishing, preserving, and living within the memory of Jesus.