Are Gnostic and pagan documents part of Roman Catholic tradition?

In the mid 1990s, I met a man online named John Wallace who impressed upon me the value of hostile corroboration. He made good use of the corroboration of Christianity that we have from ancient non-Christian sources. I also read some material in Philip Schaff's church history that left an impression on me in that context. A series I wrote on the canon of scripture several years ago has a segment about hostile corroboration of the New Testament canon, and it concludes with a quote from the material in Schaff's church history I just referred to. Ever since I came across Wallace and Schaff's work, I've given a lot of attention to hostile corroboration as a line of evidence. You can find many traces of it in my work over the years.

I often think of that line of evidence when I see Catholics and Orthodox claim that Protestants are relying on Roman Catholic or Eastern Orthodox tradition when we accept our canon of scripture, interpret it in light of ancient sources, or some such thing. They act as though anything outside of scripture should be equated with Catholic or Orthodox tradition. I know that hostile corroboration has long been a large part of what shapes my views on matters like the canon of scripture and scripture interpretation. When Bible translators make judgments about how to render the Biblical text, Biblical commentators decide how to best interpret certain Biblical passages, and so forth, they rely partly on information they're getting from Josephus, Tacitus, Pliny the Younger, Trypho, Celsus, Porphyry, archeological artifacts, and other ancient non-Christian sources. And something like a New Testament manuscript or a catacomb inscription isn't always accompanied by an extensive statement of faith on the part of the individual(s) who produced the manuscript or inscription. Think of the absurdity of suggesting that everything from Josephus to Celsus to an ancient New Testament manuscript from a largely unknown source is equivalent to Roman Catholic Sacred Tradition.

But many Protestants are taken in by that sort of argumentation. And many Catholics and Orthodox think they're arguing well when they utilize such poor arguments. That's largely because we're such a secular, trivial culture that doesn't think and talk about issues like these nearly enough.

If a Catholic or Orthodox just wants to argue that part of what Protestants are relying on is Catholic or Orthodox tradition, then that qualifier should be added upfront rather than later in the discussion. And they should justify their claim about partial dependence on their tradition and explain why that partial dependence allegedly is problematic. A Protestant doesn't have to accept, and shouldn't accept, the assumption that all or even most of the church fathers or other early Christians were Roman Catholic or Eastern Orthodox. And even if they had been Catholic or Orthodox, Catholics and Orthodox often depend on information they get from Protestant or other non-Catholic or non-Orthodox archeologists, historians, Bible translators, patristic scholars, etc. So what? All of us make our historical judgments, including judgments about matters like religion and morals, on the basis of testimony or other evidence from sources outside our church, denomination, or ecclesiastical movement. Again, so what?