how to determine when they are, and when they are not.
and thus we have checks and balance, and a jury of our peers in the most desperate of situations
one man, one priest, one written scripture cannot or will not have prejudice over the common good of the republic
if not, we would be no better than Iran
If it came down to the narrowest as you described, if the "one written scripture" we relied upon was from the Judaeo-Christian tradition, we'd still stand a good chance of being "better off than Iran" since we wouldn't be relying upon a book full of distortions of what is within what I myself recognize as Holy Writ, but instead, being from within that book (rather than the Koran, Hadith, Sunnah, or other Islamic writings) be much closer to that exact book of writings (the Bible) which has long served as a significant influence towards Western civilization's entire outlook of what is Truth.
I have it on highest authority (not simply my own opinion, or just what some man or group/organization of men have said) that what I know to be, and what is widely & generally recognized as "the Bible" is true.
The book contains truth.
That does not mean it need be taken entirely literally in wooden two-dimensional fashion throughout. For example; such as what we know as "the world" having been as it is written created in six days does not necessarily equate to that meaning only six 24 hour calendar days, as we otherwise think of "days" to consist of.
There are other ways to grasp the meaning there, without changing or else losing the meaning, yet setting aside counting those six days from merely human perspective. There is another perspective involved, which place of perspective belongs to the One who it is alleged was doing the talking.
I'll give you three guesses whose perspective that is -- but the first two guesses don't count. :^')