I don't see where Scripture says Scripture is "sufficient."
And I don't see the word "Trinity" in Scripture either. Do you? No, because the word encapsulates a concept which is never expressed in some single dry, technically precise formula. It is garnered instead from a large body of evidence. And it is every bit as true as if the theological term of art had been used.
With that in mind, let's go back over the evidence I presented earlier. First, there is context to set up. What is Paul talking about? Timothy continuinig in the faith. Based on what? Apostolic teaching from Paul, no doubt. But after Paul briefly mentions that, he dives into a much more elaborate discussion of the Scriptures, and the power they have to equip the believer, from the first movements of faith to the grand opus of a life well lived before God.
See how in verse 15 he redirects Timothy from his own apostolic ministry to the
"holy scriptures, which are able to make thee wise unto salvation through faith which is in Christ Jesus."
This is a clear statement that at minimum, the Scriptures have the power to lead Timothy to salvation through faith in Jesus Christ.
At this point we need to pause and get a basic dictionary definition of "sufficient," because this discussion will be pointless if we cannot agree on the basic meaning of the terms. From Mirriam-Webster:
suf·fi·cient adjective
: having or providing as much as is needed
Full Definition of SUFFICIENT
1 a : enough to meet the needs of a situation or a proposed end <sufficient provisions for a month>
b : being a sufficient condition
2 archaic : qualified, competent
So sufficient isn't "everything about everything," agreed? It's just "enough to meet the needs of a situation or a proposed end." And it really cannot be mistaken that Paul is telling Timothy here that the Scriptures have power "enough to meet the needs" of the "proposed end" of arriving at salvation in Christ. This is sufficiency of the most important kind.
I could rest my case here, but the passage goes on to imply an even greater sufficiency, and even uses a Greek word which translates well as "sufficient."
Consider verses 16 and 17 as a unit:
2Ti 3:16-17 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness: (17) That the man of God may be perfect, throughly furnished unto all good works.
The word "perfect" in verse 17 is artios. Here is the lexical description:
STRONGS NT 739: ἄρτιος
ἄρτιος, ἀρτια, ἀρτιον (ἈΡΩ to fit (cf. Curtius, § 488));
1. fitted.
2. complete, perfect (having reference apparently to 'special aptitude for given uses'); so 2 Timothy 3:17 (cf. Ellicott at the passage; Trench, § xxii.). (In Greek writings from Homer down.)
Now, I am not rying to be erudite. I'm trying to make a case well enough that you can see how this idea of the sufficiency of Scripture flows from Scripture quite easily and naturally. To that end, I would like you to consider what it means to have a resource that has, without further additions, the power to make you "complete, perfect," or having "special aptitude for given uses," all with respect to your faith in and relationship with Jesus. Would you say that resource was "enough to meet the needs of a situation or a proposed end?" If not, then you are saying Paul is wrong by omission, because he left out things we need, while clearly suggesting Scripture could get us there. But if you agree that Scripture can accomplish what Paul says it can here, then you do believe in it's sufficiency to the intended purpose, faith in Christ and godly living.
Now you mentioned 2 Thess 2:15 as a proof text that extra-biblical traditions were considered necessary even by Paul. However, that passage does not say that. Let's look:
2Th 2:15 Therefore, brethren, stand fast, and hold the traditions which ye have been taught, whether by word, or our epistle.
Your theory requires a distinct tradition that is additive to the New Testament body of doctrine. Paul here is saying that the tradition they now hold came to them both by Paul's spoken word and by Paul's epistle. But there is nothing to suggest a difference in content. The tradition was contained in both Paul's spoken words and his written words. Therefore, even though we no longer have Paul speaking to us in person, we can still access the tradition he gave the Thessalonicans through his epistle. Both were channels of the same message. Paul never suggests he left something critical out of his writings, something essential to salvation that would not appear for centuries to come, only to be rediscovered and reasserted by some lonely monk. That sounds more like a Dan Brown novel than Paul.
It also helps here to remember what the struggle was in this epistle. There were false teachers going about disturbing the flock with terrible theories that they had missed the second coming of Christ and the resurrection as well. There is even an inference back in verse 2 of this chapter that someone had forged a letter in Paul's name propagating this cruel and heartbreaking falsehood. Pauls is telling to stand fast in the eschatology he has just shared with them in this epistle, which message was consistent with what he had told them in person. So the focus is not on the notion of a separate additive oral tradition, but on the sameness of what Paul had told them in person and what he was telling them now by letter.
Well, it's getting late, and I'm nodding off. Going to bed now.
Peace,
SR