The very selection of canonical vs non-canonical books is tradition. And I rely on that heavily to judge the relative reliability of different documents. I take those universally canonical books (Protestant bible) as 100% reliable, those considered canonical by a portion of Christians as strongly worth paying attention to, and everything else to be treated with more caution. On what basis do I justify this? Honestly, only tradition.How much weight do you give the traditional interpretation of verses?
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In the case of support for a doctrine or position coming from non-canonical books, do the verses carry as much weight as those in the regular biblical books?
In the non-canonical books, you must use discernment to see what is more likely to be reliable and profitable, and what is less reliable. For instance, Tobit is awesome for studying marriage - whether it's 100% historical fact I couldn't tell, but it describes Hebrew marriage in more detail than anything in the Protestant bible. Bel and the Dragon contains a great and plausible account of a probable dinosaur - but whether Daniel actually was thrown to the lions a second time like it suggests I find less likely. The Gospel of the Birth of Mary reads like pure fiction. While the letters of Clement to the Corinthians seem as sound as any New Testament writing bar a couple of very minor matters of question.
But how do I tell this? Only by comparing what they say to the canonical books. And I accept them as the gold standard due to tradition. I cannot escape tradition.
Tradition is the accumulated wisdom of generations of Christians who have gone before us. It is not to be discarded lightly. If in doubt, I'd run with the traditional understanding (or at least a traditional understanding if there are multiple options...), because it's more likely to be correct than anything I come up with by myself.
But if the text clearly contradicts the tradition, then go with the text.