One standard type of argument in Christian apologetics is to infer that if Scripture is reliable on those occasions where it can be corroborated, then this creates the presumption that it’s reliable on other occasions when it can’t be corroborated. If a man is accurate whenever you can verify his claims, then he’s trustworthy in general.
After all, it’s something of a coincidence that corroborative evidence survives. For the survival of corroborative evidence is a fairly random, hit-n-miss affair. A writer couldn’t predict what corroborative evidence would survive, then make sure his testable claims were accurate while fibbing the rest of the time. Given the haphazard nature of the corroborative evidence, if a writer is accurate on just those occasions when we happen to have corroboration, then it’s unlikely that he would be inaccurate on just those occasions when we don’t have corroboration.
Mind you, corroboration isn’t always a straightforward process. For instance, it can be complicated to correlate textual place names with extratextual sites:
The most basic problem is that in a great many cases several names are possibilities for a particular site. On the other hand, there may be several candidates for a name that has been historically recorded…On the other side of the issue, there are tells that could be the remains of any number of historical sites. J. Currid, Doing Archaeology in the Land of the Bible (Baker 1999), 57.
Yet Catholic epologists deploy the opposite argument. Take Munificentissimus Deus. They take the position that infallibility only extends to the formal definition. To the dogma, and not the pope’s supporting arguments. On this view, the historical and exegetical arguments of Pius XII could be balderdash, but his sheer declaration of the Assumption is selectively shielded from error.
Put another way, all of the pope’s testable claims could be dead wrong, but when he happens to make an untestable claim (the assumption of Mary), that particular claim is true. It’s like saying that even though a man is a chronic proven liar, he’s suddenly credible whenever he happens to make unfalsifiable statements.