It is the right of nontheists to require theists, generally, to agree on a single scripture and a single interpretation of that scripture before entering upon any discussion of possible scriptural validity. For, until almost all scripture-believers can agree on the truth of one scripture, no scripture held to be universally valid even exists to be deemed fit for discussion as a candidate for being a true account, and hence 'the one true scripture.'
It is well observed that people tend to join the faith hewed to by their parents and most prevalent in their geographic setting. And it is fair to contend that most people who have been brought up in a particular religious tradition, and have been brought up reading the scriptures of that tradition, have not done much in the way of reading the scriptures of other religions. The vast majority of Muslims have never read the Bhagavad-Gita or the Book of Mormon; most Christians have not read the Book of Mormon, the Qu'ran, or any part of the Vedas; most Hindus have never perused the Old Testament. And indeed we may go one further than that and observe that many, if not most, adherents to any one scriptural faith have never fully and thoroughly read their own scripture, and have instead tended to rely upon their religious leaders to convey what meaning ought to be gleaned from it.
But contrariwise, I have personally known Muslims who read the the Bible, Old Testament or New, and at the end of the day expressed the sentimentation that it was an interesting book, but was in error while the Qu'ran was not so. And I have known Jews who read the New Testament and came to the same conclusion. And I have known Hindus who read both Qu'ran and Bible and found both to be errant accounts. And, naturally, I have known Christians who read the Qu'ran, the Book of Mormon, some of the multitudinous Vedas, or other scriptures, and have found all the other works to be in error, with only their Bible being true (and indeed only their particular translation being true). The common denominator amongst all of these was this: people who were born and raised into a particular tradition were inclined to believe that the superior scripture was whichever one there parents or peers had told them about. To this I will add that I have met in my time a small selection of religious converts; those who converted from Christianity to Islam or to Judaism or to Mormonism or to Wicca, as well as from Islam to Christianity, or from Mormonism to Christianity. The proportion of people who have changed faiths in this manner is small overall, and dwarfed by the number who have converted from theistic faiths to nontheistic traditions -- to Deism and Pantheism and Pandeism, and, naturally, to Atheism and generic incarnations of Agnosticism.
But this process seems counterintuitive, for if there is indeed one 'true' scripture, if one empirically stands above the rest, then honest promoters of that one ought to be able to convince honest and studious representatives of all other faiths of the truth of that one scripture. And it is inconceivable that faith traditions with millions of adherants ought not to have substantial numbers of the honest and the studious. And, those studious representatives being converted to the veracity of the one true scripture, the rest of their cobelievers ought quickly to fall into the same belief. For every scripture assigns consequences to the continued teaching of falsity, so if the most dedicated of Muslim Imams were to be convinced by logical explanation of the text that Tibetan Buddhism were true -- and that he would suffer a poor reincarnation and be distanced from Nirvana by continuing to lead others into Islam -- then rational thinking would offer no choice but to convert to that strain of Buddhism, and explain as much to his followers (no matter the consequences to himself of breaking with his previous practice).
But the vast majority of people in the world have rejected each of the scriptures and interpretations of them. Billions of people have rejected Hinduism, billions have rejected Islam, billions have rejected Christianity as a whole and billions more have rejected each of its individual branches, billions have rejected Sikhism. Now it is true that billions, perhaps billions more, have rejected nontheistic faith traditions such as Buddhism and Atheism and Pandeism, but these traditions have never claimed to have God-written books which muct be taken as true on pain of eternal torment. Buddhism for example simply has the observations of a man, the Buddha, self-proclaimed as nothing more than a man who reached a state of understanding. Buddhism teaches that all the scriptural religions in which men believe are simply part of the web of desires which humans erect to justify their continued wallowing in the suffering of want. Atheism and frequently Deism account for scripture-based religions in reductionist terms, as purely psychological and sociological phenomena. Pandeism accounts for scriptural religions as being, for the lion's share, manifestations of man's efforts to comprehend the incomprehensibility of the underlying, unconscious mind of our Creator.
And it is true as well that Buddhism, for example, has the writings of the Buddha, and other traditions have particularly revered writings such as Thomas Paine's The Age of Reason is to Deists. But by definition none of the non-scriptural traditions have or allow for a scripture, a written volume contending to have been inscribed or inspired directly by an intervening deity. And so we reach the crux of the argument, which is that amongst those who believe it possible for their deity to have had an intervening hand in the writing of a book, and who believe that the occurrence of this intervention is evident in but a single one of the many, many books for which this claim is made, then the truth of this assertedly evident intervention would come through the acceptance by most all scripture believers of the one scripture which was 'true,' and of the one most 'true' interpretation (and, even version or edition of that scripture, if more than one exists).
What is left:
Naturally, it is always conceivable that some small number of especially gullible types will be drawn into belief within a cult having a wholly implausible scripture. And so, it can not be required that a single work be recognized by the entirety of the theistic population, as people may reject a scriptural account for irrational reasons (irrespective of whether a rational person might reject such an account anyway). But once we get into faiths having hundreds of thousands or millions of followers, the equivalence of scripture evens out. For such population cannot be maintained without having followers within them who are free to make whatever inquiries they wish into the saliency of the scripture at issue, and inevitably having amongst that selection some whose perception and intellect tower above the average both within that faith and for humanity in general.
Nor is this itself an argument that no scripture is or can be true. It is instead simply an observation, that if one scripture were true, and were rationally demonstrable to be so, then all rational believers in the saliency-at-all of scripture ought to quickly come to acknowledge its truth -- and to do so before those who do not even believe in scripture may be expected to do so. And even if this truth were one only spiritually perceptible, if there were one scripture which was true in that sense it ought to be perceptable in so fair and honest a way that honest and studious persons of a spiritual bent coming from any background would conclude it to be more true than that to which they had heretofore adhered.
It is certainly possible to discuss generic -- that is, nonscriptural -- questions as to asserted proofs of God, even absent an acceptible universality of scripture. The Cosmological Argument and the apparent fine tuning of our Universe, the experience of the ethereal as reported by the mystics, all of these are debateable as proofs of divine creation even if no holy book has any word of truth in it. But if those already believing in the saliency of scripture are unable even to convince one another of the final correctness of one amongst that multitude, then no scripture yet written can be claimed to be the one to which nontheists must accord consideration as having a sufficiently persuasive effect to merit really any drop of nontheistic consideration whatsoever.