My Bible has introductions to the various books of the Bible, giving historical context, etc. One thing that often happens, is that it calls into question the true author of the book. For example in 2 Peter, it says that scholars today do not believe that the letter was actually written by Peter, but by one of his followers many years later.
If this is true, then how reliable is the book itself? If the person to wrote 2 Peter says that he is Simon Peter, and that he was an eye-witness to the Transfiguration, but he is actually just a disciple of Peter, then I start to question other things about the letter.
What are we to believe? If the author can take liberty to act as if he is someone he isn’t, then why is everything else trustworthy?
The same is said of the latter portion of Isaiah.
Jon
The doubt about authorship is what kept the book out of the canon until quite late - so this is nothing new.
The letter may be Peter’s - & it may not: there is simply no way of knowing. It doesn’t ultimately matter, because the primary author of the books - all of them - is God; so certainty about the exact identity of the human authors is not essential to acceptance of the books as “sacred and canonical”. They are inspired by God, & recognised as such by the Church, & that is what counts. The letter has long been recognised to be related to Jude in some way; the passage in chapter 3 directed against those who doubt the Second Coming of Christ certainly reads as though it comes from a time when people were beginning to weary of waiting for it; & that weariness seems not to have set in during St. Paul’s lifetime.
The author of Ecclesiastes sounds as if he ought to be Solomon, but the mention of other “kings in Jerusalem before me” weakens the impression created by the first verse. As for Isaiah, most of 1 to 39 is his; 40 to 66 seem to require a date round about the end of the Exile & the fall of Babylon, in 539; 46.1 (for example) fits in admirably with a later date, but far less well with a date in the 700s. The mention of Cyrus can hardly be from the Isaiah who was a contemprary of Ahaz & Hezekiah. There are good reasons on both sides of the argument - the book is still Scripture, no matter what the date of its parts.
It may be worth emphasising that, when people raise these issues & uncertainties, they do so because the Biblical texts create the problems in the first place; or else the detail of the letter of the text is brought into contact with something outside the Bible. The point is, that it is unfair, unjust, & unnecessary to suppose the worst of those who raise such questions: yet this sort of thing is supposed every time there is one of these threads

These positions, however surprising, are adopted for very good reasons, & nothing is gained by blackening the characters of those who adopt them; their critics, who are so quick to attack them, seem never to have anything constructive to offer. If a suggestion is unfounded, let it be replaced by one that is well-founded. But that never happens - complaining is easy, unlike being constructive.