Hi Angus,
The doctrine of the Sufficiency of Scripture is explicitly about what we can understand from Scripture. The doctrine has always assumed (I recommend
The Shape of Sola Scriptura on this topic.) that interpretation is governed by the church's 'rule of faith'. Yes, it does oppose the Magisterium but it does not oppose tradition, broadly understood, it is important to clearly recognise this.
I'm not saying we can't get value from extra-biblical information, sometimes it helps us strip away our own preconceptions but it should never be relied on to shape our interpretation of Scripture!
Angus, I think you've overlooked my point about translation when you wrote this:
For a start the Bible wasn't written in English so we depend on others to translate the source texts, and to know which source texts are the most reliable. These translators also need a good understanding of the cultures in which the Scriptures were written (often gleaned from extra-biblical sources) to better understand and accurately translate these texts into modern idioms.
I said this in my earlier response to you about translation:
"'Sufficiency of Scripture' is not about translation but is about the question of what does "Scripture teach us about faith and godly living?" The purpose of Scripture is so that we may know God and all that that involves. It is entirely sufficient for that purpose. Furthermore the universal applicability of faith and godly living demonstrates the principles of Scripture transcend their original historical and cultural context."
Are you really suggesting that Scripture has to give us permission to use archaeological information to better understand the contextual cultures?
Absolutely!
Why would we use any other method to understand Scripture? Scripture is self asserting, everything we know and believe about Scripture is from Scripture itself, we have no other epistemological basis. (This is essentially Peter Adam's main argument in his latest book.) So in a sense you haven't fully responded to my earlier challenge.
Anyhow, even the Scriptures use non-canonical texts at times to clarify Scripture (see Jude 1:14-15 & "Book of Enoch").
Great point to which I would add that Paul uses references to paganism, Proverbs includes Egyptian wisdom and various other examples. However it is important to note that each of these are integrated into the cannon by the inspiration of the Holy Spirit. If fact you could argue everything in Scripture is human, or non-canonical until God brings into His revelation. A better way of viewing things would be to ask how does Deuteronomy interpret Exodus, how do the prophets interpret the Law, how does Jesus interpret the OT, how does Paul interpret Jesus and the OT? What is the overall pattern being display here and how should our interpretation mirror that pattern?