I believe the evidence that may very well seal the legitimacy of orthodoxy as historic Christianity would be through the Pastoral epistles. The Pastoral epistles have the potential to destroy any argument against early orthodoxy - and Bauer was one of the few critical scholars who actually noticed this. Therefore, for this reason to avoid the dilemma the pastorals would cause, Bauer went on to date the Pastoral epistles to the mid-late second century as a Roman response to Marcion (ca. 86-160) whose teachings peaked in the 140s in order to restore proto-orthodox trust in the Pauline corpus.
If Bauer were to acknowledge the pastorals as (1) authentic or (2) belonging to an earlier period, his entire argument of a lack of any sort of orthodoxy would fall through.
The epistles are focused very much so on defining orthodoxy, and asserting its historical and apostolic dominance over the later heresies that arose. The famed 1 Tim 3:16 establishes the Old Testament as canon - evidently trumping the Gnostic and or Marcion claim to apostolic heritage. We know that the Jesus movement arouse from the midst of the Palestinian Jewish context and we know that Jesus' unique relationship with the Father was built on the OT. Furthermore, it was the OT which establishes Jesus' ministry and position in light of Messianic expectations.