In Scripture, we have everything we need to be "complete, thoroughly equipped for every good work" (2 Timothy 3:17).
Please notice that the decisive word everything here is your word, and does not form part of the Scriptures. Yet it whether the written Scripture has "everything," is precisely the question which is under dispute.
You can't prove it by inserting the decisive word "everything" in there, yourself! And while Scripture itself says that all inspired writing "is useful," it does not say that the inspired writing is the sole rule of faith. In fact, Scripture itself indicates a need for more than Scripture alone: I'm referring to what was handed down orally by the Apostles.
2 Thessalonians 2:15 So then, brethren, stand fast, and hold the traditions which ye were taught, whether by word, or by epistle of ours.
Please read the two verses immediately before 2 Timothy 3:16-17 :"But as for you, continue in what you have learned and have firmly believed, knowing from whom you learned it, and how from childhood you have been acquainted with the sacred writings which are able to instruct you for salvation through faith in Christ Jesus" (2 Tim. 3:14–15).
Notice that in v. 15, Paul is referring to the scriptures which Timothy was taught as a little boy. (Some translations say "infant," some say "babe.")
Most of the the various letters, papers and books of the Christ-inspired NT authors were not even written at the time of Timothy's childhood, let alone scrutinized by the Church,sorted out from other writings which were of dubious authenticity like the non-canonical "Gospels,", and assembled into the canon of "The New Testament." So St. Paul is referring to the writings of the Old Testament; and I'm sure you'd agree that he is NOT saying that only the Old, and not the New Testament, is necessary for one to be "complete, thoroughly equipped."
Paul tells Timothy to continue in what he has learned because he knows from whom he has learned it, Paul himself: that is, the oral teaching which the apostle Paul had given Timothy. When the passage is read in context, it becomes clear that Paul is teaching the importance of BOTH the written and the oral (unwritten) parts of apostolic tradition.
Paul says that much Christian teaching was handed down by word of mouth (2 Tim. 2:2). In another Epistle, he instructs us to "stand firm and hold to the traditions which you were taught by us, either by word of mouth or by letter" (2 Thess. 2:15).
This oral teaching was accepted by Christians, just as they accepted the written teaching that came to them later. "So faith comes from what is heard, and what is heard comes by the preaching of Christ" (Rom. 10:17). It is un-Biblical to limit "Christ’s word" to the written word only or to suggest that all his teachings were reduced to writing. It certainly doesn't say that in the Bible.
The oral teaching of Christ would last until the end of time. "’But the word of the Lord abides for ever.’ That word is the good news which was preached to you" (1 Pet. 1:25). Note that the word has been "preached"—that is, communicated orally. It is this which will "abide forever." It would not be totally replaced by a written record like the Bible (reinforced, yes, but not replaced), and would continue to have its own authority.
This must be obvious when you recall that, of the Twelve who were sent by Christ to plant His Church, only five--- Matthew, John, Peter, James, and Jude --- wrote Gospels or Epistles which were eventually recognized by the Church as being part of the canon of Sacred Scripture. Seven others --- the "lesser" James, Andrew, Bartholomew, Philip, Simon, Thomas, and Judas' successor, Matthias -- were sent out equipped with nothing in terms of a written New Testament, and with only their own oral, apostolic testimony as the basis of their mission.
These men carried out Christ's Great Commission to preach, teach, and baptize all nations , at least at first without written Gospels or Epistles. Or do you think that these men journeyed to places like Ethiopia, Edessa, Assyria, Cappadocia, Bithynia, Byzantium, Thrace, Macedonia, Thessaly, and Achaia --- all sites of apostolically-founded churches --- and said, "Um... er... well, I'm not equipped to say anything, because I have little or nothing in writing"?
2 Thessalonians 2:15 So then, brethren, stand fast, and hold the traditions which ye were taught, whether by word, or by epistle of ours.