I know this is very basic. Tooth grindingly basic. Agonizingly obvious. Belileve me, I know. But having had to explain this concept a number of time in the past few days I feel compelled to just list out a few reasons so I can simply direct people here rather than repeat myself.
Simply stated, you cannot use the Bible, Quran, Torah, Talmud, Book of Satan, or any other sacred text to prove that what that book says is true. Why?
Let us assume arguendo that the <Insert Allegedly Sacred Text> is internally consistent and free of logical contradictions. Does internal logical consistency bequeath external truth?
Internal logical consistency is the hallmark of good writing and good storytelling but it in no way either mandates or implies external truth.
As an example, the Iliad, the Lord of the Rings, and Harry Potter are all very internally consistent. Indeed, they are more internally coherent and logically consistent than <Insert Allegedly Sacred Text>. Their internal consistency in no way implies that they possess any manner of external truth. Achilles is not really invincible and the gods are not really taking part in a regional conflict over Helen of Troy, The Dark Lord Sauron does not present a clear and present danger to the state of Gondor, and Lord Voldemort is not actually waging a shadow war against the Ministry of Magic in the greater London area.
Internal logical consistency is a NECESSARY condition for external truth, but not a SUFFICIENT condition for external truth. Even assuming without argument that <Insert Allegedly Sacred Text> is internally and logically consistent, one cannot arrive at the conclusion that it is externally true based solely on that consistency.
The ONLY reasonable conclusion one can reach based solely on internal consistency is that <Insert Allegedly Sacred Text> says X, therefore <Insert Allegedly Sacred Text> says X. One CANNOT reach the conclusion that <Insert Allegedly Sacred Text> says X, therefore X is True. To reach such a conclusion requires either direct evidence that X is true, or that <Insert Allegedly Sacred Text> is logically consistent, free of contradictions AND can be shown to be consistent with external evidence.
And <Insert Allegedly Sacred Text> makes some pretty wild claims. Angels, Demons, Dragons, Talking Snakes, Global Floods, Collection of EVERY animal on Earth, Seas Parting, Resurrection from the Dead, Bodily Ascensions Into the Sky, Sun Stopping in the Sky, Plagues, Food Falling From the Sky, Alchemy, and all manner of other miracles. <Insert Allegedly Sacred Text> states that these things occurred. Is that sufficient?
When someone claims to have seen Bigfoot or the Loch Ness Monster, or claims to have been abducted by aliens, they are making a fairly incredible claim. A claim not at all dissimilar to those made in <Insert Allegedly Sacred Text>. Does any reasonable person simply accept their statement? Of course not. Any reasonable person demands evidence, corroboration, proof. And when they produce an incredibly blurry video that shows some unidentified movement do we accept it? Of course not. Because extraordinary claims require extraordinary proof. Personal testimony is insufficient. And hearsay even less so.
Which is why we demand evidence, corroboration, support, proof.
The reality is that the veracity of <Insert Allegedly Sacred Text> is only taken as a given by those who have already decided that it is unerringly true. Even if one is to assume without argument that <Insert Allegedly Sacred Text> is logically and internally consistent, which is a huge and utterly unwarranted concession, said consistency is still useless from an evidentiary standpoint unless it is corroborated, supported, evidenced from external sources. So spare me the endless recitation of Timothy II 3:16. The fact that the Bible says it was breathed from God doesn't make it so any more than the Quran's claim that it was given by God to Muhammad through Gabriel.
You can't use <Insert Allegedly Sacred Text> to prove <Insert Allegedly Sacred Text>.