Stephen said
Every method employed has methodological assumptions, which are not merely willy-nilly assumptions but based on the considered results of prior, communal study.
Whereas I enjoy a position of absolute objectivity from which I can judge with perfect equanimity both the living and the dead. (I am given to understand I have a special role in the Parousia so treat me right.)
Nothin’ so highfalutin’ Robert.
The assumptions of which I speak are rather specific and functional. It seems to me a case built for dependence must begin with the assumption that John had no other literary or oral sources than the Synoptics. Because if he did how could you ever determine that any similarities (or differences) were not the result of these unknown sources? As I said this is the corner Robyn Faith Walsh paints herself into. She discounts oral sources for the ministry of Paul so the composer of Acts must know Paul’s letters. How else could he have ever known about Paul otherwise? But then how do you explain the fact he gets Paul all wrong and screws up the timeline?
This it seems to me is the danger of positing a single literary tradition. You almost have to assume it’s true before you can demonstrate it’s true. Am I the only one for whom this is an issue?
  

If we credit at least the 7 “least disputed” letters of Paul as composed by Paul, then Paul was involved in the establishment of multiple faith groups in multiple cities.

How is it considered unlikely that members of a faith group originally evangelized by Paul would share stories about Paul with a visiting teacher and scribe? Since discounting “oral sources” for the ministry of Paul seems to require current members of faith groups who can trace their founding to Paul remaining grimly close mouthed if such a person is visiting.

But there is a question of to whether there is a difference that makes a difference if John knows the synoptics in the sense of having copies at hand or if John knows smaller texts in circulation that were directly used in the synoptics … and of course, it would be a false dichotomy to set “oral sources” in one box and “texts” in another box, since a literate teacher can set what they have heard on paper, and someone who is illiterate may well be able to learn a speech or parable or illustrative story that has been set down on paper and read out by a literate member pass it along orally.

If the author of “Luke” is not blatantly lying about basing the gospel on a range of sources, then it is not entirely implausible that “Matthew” is doing similarly with an overlapping range of sources, then so long as John is not adopting the core literary structure of one of the “amplified” versions of Mark, it might not be possible to distinguish between using a synoptic directly and using some of the synoptics direct sources.

In that case, there’d be no point in having a distinction which available evidence cannot be used to interrogate, so if the narrowest statement that evidence can be marshaled to support is “John know the synoptics and/or their direct sources”, then that would be the empirical question to argue for or against. 

Stephen said
If we credit at least the 7 “least disputed” letters of Paul as composed by Paul, then Paul was involved in the establishment of multiple faith groups in multiple cities.
Prof Walsh even disputes this viewpoint which she refers to as the idea of the “hero Paul”.  See a couple posts Walsh made at this very site last year.
Paul and the Anachronistic Origins of Early Christianity – Part 1 by Dr. Robyn Faith Walsh   
Paul and the Anachronistic Origins of Early Christianity – Part 2 by Dr. Robyn Faith Walsh
It speaks well of Prof Ehrman that he is willing to platform a scholar whose work presents a sustained critique of his ideas. Read the comments as well.  She expands on a couple points as the result of questions. 
I don’t dispute that Acts is an attempt to create an “origin” story for the Christian community.  It attempts to establish a connection between the later, largely gentile churches and the original Jerusalem community that was probably destroyed as the result of the Roman destruction during the revolt.  (We know from Paul himself that there was a non-Pauline gentile ministry.) But this is the classic “chicken or the egg?” situation.  Did the author of Acts use Paul as his “hero” because he was already well-known and influential?  Or did the author take Paul and exaggerate his fame and his influence?  Ya pays yer money and ya takes yer chance. 
What I have most difficulty with is the dependence she must posit of the gospel writers on Paul’s letters.  The author of Luke/Acts knew about Paul but did he really know Paul?  There are similarities with to Mark’s soteriology but differing Christologies. And I asked her about Matthew but she didn’t really answer the question.  (Certainly Matthew and Paul shared an intellectual milieu but they would have scratched each other’s eyes out!)  
Still, if you haven’t read Prof Walsh’s book, do so.  She has lots of provocative things to say and it made me stop and do some thinking. 
  

I only said “played a role in”, which is a bit less heroic than the “Hero Paul” story. I very much like Dr. Tabor’s view of Paul’s missionary voyages as Paul getting run out of town by his opponents by various of the other strands of the early church, many of them laying claim to founding by “real Jesus disciple apostles”, and moving on to another location further away from the Jerusalem church to start over, so the count of faith communities that he played a role in founding may well have been larger than a count of the faith communities that were still largely Pauline at the time of his death.

So I read Acts of the Apostles as, in part, an attempt to paper over the differences between the still-Pauline faith communities and other prominent gentile faith communities, an ex post facto resolution of conflicts which were very much unresolved at its time of writing. And Acts of the Apostles was adopted widely enough to later be canonized as opposed to a variety of other Acts because the resolution of the conflict proved in practice to be acceptable enough that it could be used by both Pauline and Petrine gentile faith communities.

The emphasis on Peter in Acts of the Apostles and glossing over the first “Bishop of Jerusalem”, even while clearly recording James as the decision maker when Acts composes a grant of apostle to the gentiles to Paul, suggests strongly that the Jacobite faith communities were ebbing in influence at the time of composition of Acts of the Apostles, but James position as the original head of the Jerusalem church was still too widely known for him to be written entirely out of the account.

During the semester, my free time involves more a watching videos level of energy than a reading demanding books level of energy, so if I go from watching some of her videos to picking up Dr. Walsh’s work proper, it will be next month.