2 Thessalonians 2:15 (NASB):
15 So then, brothers and sisters, stand firm and hold on to the traditions which you were taught, whether by word of mouth or by letter from us.
What traditions is Paul talking about?
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Paul is contrasting false teachings with the true gospel in this chapter--he wants them to hold on to the true gospel that he (and other missionaries) have taught them.
He makes reference to their receipt of the gospel in the prior verse:
14 Whereunto he called you by our gospel, to the obtaining of the glory of our Lord Jesus Christ.
And he references what he personally taught them (see v5)
Warnings of false doctrine
Contrast this with the warnings of false doctrine that inundate this chapter:
Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition (v 3)
For the mystery of iniquity doth already work (from v7)
And then shall that Wicked be revealed (from v8)
Even him, whose coming is after the working of Satan with all power and signs and lying wonders,
And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved. (vs 9-10)
That they all might be damned who believed not the truth, but had pleasure in unrighteousness. (v12)
Paul is warning them: apostasy & heresy are coming, be on your guard. And he's vouching for the truth and security in the message he has shared, whether in person (see Acts 17:1) or in writing (1 & 2 Thess.)
In a word, "the faith once and for all delivered1 to the saints" (Jude 1:3). This faith comprises many "traditions" (2 Thess 2:15), all of which concern what Christians are to believe (doctrine) and how (practice, sources).
An example of what to believe is that Christ died for sins. And example of how to believe that, or a way to express belief in, and partake in that, was to celebrate the Eucharist each week, as the "witness" of every century shows was an Apostolic institution:
2 Timothy 2:2 And the things which thou [Timothy] hast heard of me by many witnesses, the same commend to faithful men, who shall be fit to teach others also.
That is, the Apostles taught the Christian faith not only via letters (inspired, or otherwise) — "our letters" — but also "orally" (ibid.), including doctrine, and practice.
It's impossible that St. Paul meant here that there were a secret set of traditions only related to the Thessalonians, and not the rest of the Church. Rather, he means hold to Apostolic tradition in whatever form, written (Epistles), or otherwise (in person teaching).
What the Apostles commit in teaching, "whether by one of our letters, or orally," is to be adhered to by all Christians. Just as 'which books are inspired' is a tradition, so are the others equally, such as how to perform the sacraments or mysteries, such as the Eucharist, which has clear common elements common in all the Apostolic churches, even though variations exist (these actually accentuate and bring into focus what was fundamental, by their variations on the same, just as 'contradicts' in the Gospels often solidify the historicity of the thing being 'contradicted').
In other words, Paul is referring to tradition in general — "the traditions which you were taught" as containing all the truths of Christianity — and is not referring to any particular ones.
1 Literally, "traditioned."