In respects to answering the question of "How does one know that the 27 NT books are all inclusive (nothing left out and nothing falsely included), non-Catholics often cite the 39th Festal Letter of Athanasius (367 AD), which we all know is the earliest citing of all 27 books. While this document alone is not the end of the evidence trail for most, I’m confused by what seems to me a conditional trust in the source of the document (Athanasius and more generally the Church to which he belonged). In other words, non-Catholics will trust Athanasius in respects to this document, but they do not trust him in respects to beliefs that he held, such as the Eucharist.
This brings to mind the question: If the Holy Spirit guided Athanasius to accurately represent the NT canon, is it logically coherent to hold that the Holy Spirit did not guide Athanasius in respects to his beliefs on the Eucharist?
One non-Catholic response that I have been able to think of is that we unconditionally trust God, but conditionally trust men. However, I don’t see this answer convincing, because I struggle to see it any different from simply responding to the ‘how do you know question’ by avoiding history and saying ‘God’.
I do see the general consensus of the Church Fathers’ documents/beliefs a plausible response to know that the 27 NT books are all-inclusive, and but at the same time, this seems to be a plausible argument for the Eucharist as well.
Would like to hear some different perspectives on this thought process - Thanks!