The problem with taking 2 Pet. 1:21 as a support for the inspiration of Scripture should be obvious for anyone who reads it slowly and deliberately: It does not refer to the inspiration of Scripture per se but rather to the inspiration of prophecies within Scripture (v. 20). 27 Nothing in the verse implies that epistolary and narrative writings found within Scripture are also inspired. 28 The discussion leading up to 2 Pet. 1:21 has to do with the assuredness of the Lord’s word, as delivered in a prophetic mode. Nothing in this verse can reasonably be construed as a statement about the nature of Scripture itself.
The Invention of the Inspired Text: Philological Windows on the Theopneustia of Scripture (The Library of New Testament Studies, 640)(Pages 110-111).
Take a look at the passage:
16 When we told you about the power and coming of our Lord Jesus, the Messiah, we did not follow any clever myths. Rather, we were eyewitnesses of his majesty. 17 For he received honor and glory from God the Father when these words from the Majestic Glory were spoken about him: “This is my Son, whom I love. I am pleased with him.” 18 We ourselves heard this voice that came from heaven when we were with him on the holy mountain. 19 Therefore we regard the message of the prophets as confirmed beyond doubt, and you will do well to pay attention to it, as to a lamp that is shining in a gloomy place, until the day dawns and the morning star rises in your hearts. 20 First of all, you must understand this: No prophecy in Scripture is a matter of one’s own interpretation, 21 because no prophecy ever originated through a human decision. Instead, men spoke from God as they were carried along by the Holy Spirit.
While John C. Poirier is correct that the passage doesn’t state that every proposition within the Bible in its context is correct, it is the intention of the author that maintains we may trust these things (prophecy, wisdom, etc.) because it is divine in origin. Peter is connecting the notion of veridicality with God as the apex of what can be trusted by man.
This is extended by implication to all other words spoken by God, as Peter quotes Matthew 3:17 as authoritative because he recognized the fact that these words result from God. Hence they didn’t follow fables. The authors cared about the truthfulness of what he wrote and believed. We can trust these words if they come from divine origins.
Are we to suppose the author’s own words are not thought to be authoritative? Given Poirier’s own criterion, this is neither a prophecy nor a matter of legality. So, it can be discarded as an ancient opinion.
It is more likely that “prophecy of scripture” means what it appears to mean, and that v. 21 tells us only that these prophecies are inspired. The point of the verse is essentially that the prophecies preserved in Scripture are real prophecies.
The Invention of the Inspired Text: Philological Windows on the Theopneustia of Scripture (The Library of New Testament Studies, 640)(Page 111).
This seems forced. Why think these divides were used by ancient writers to divide the text into veridical and non-veridical parts? This is liberal anachronism at best. Notice that Poirier misinterprets the text. The text doesn’t say only certain prophesies, or that only prophecies are inspired. It states that prophesies are inspired, but this doesn’t deny the inspiration of that which surrounds prophesy.
Furthermore, the different strains of evidence one uses to convince people that the Biblical writers thought of these entire books as something that came from God:
vi) In the organic theory of inspiration, especially with a strong doctrine of providence, inspiration doesn’t require a special state of mind. God can prearrange all the variables so that a Bible writer will naturally choose certain words to express correct beliefs.
vii) Moses was the paradigmatic prophet, yet he was not typically a recipient of visionary revelation (Num 12:6-8).
viii) Paul ascribes verbal inspiration to his teaching (e.g. 1 Cor 2:13; 1 Thes 2:13). Even though Paul was a seer, we need to distinguish between visionary revelation and verbal inspiration. Once again, it seems to be a two-stage process. His written word was ever bit as authoritative as his spoken word.
ix) When quoting the OT, the author of Hebrews attributes all statements directly to God, even though God wasn’t the immediate speaker. That equivalence only makes sense given verbal inspiration.
x) Jesus, the apostles and/or NT writers prooftext their claims by appeal to OT books without regard to genre. So inspiration was not confined to the prophetic genre.
http://triablogue.blogspot.com/2014/06/plenary-verbal-inspiration.html
This also fits with why OT Jews held that certain books belonged to the canon of scripture. Why hold certain books as sacred and different if only certain parts are authoritative?
The Council 