1 Corinthians 11:2 - I commend you because you remember me in everything and maintain the traditions even as I have delivered them to you.2 Thessalonians 2:15 - So then, brethren, stand firm and hold to the traditions which you were taught by us, either by word of mouth or by letter.
2 Thessalonians 3:6 - Now we command you, brethren, in the name of our Lord Jesus Christ, that you keep away from any brother who is living in idleness and not in accord with the tradition that you received from us.
Maybe I'm saying something that's already quite clear to you, but here Paul uses three phrases which specify "What tradition?"
Paul is saying that there are traditions from him and/or from him and the other apostles ("us")
And he specifically says these traditions (Apostolic Traditions) have the same authority, whether they are "by word of mouth" or "by letter."
So he's not talking about "any old tradition." He's talking about specific things said or written from himself and the other Apostles. That's why they're called "Apostolic," and why, to distinguish them from other traditions, these particular ones are often spelled with a capital-T in languages which use upper-case and lower-case letters.
(Note that neither the Hebrew nor the Greek in the Apostolic Age had both upper-case and lower-case letters, but we do in English and some other languages.)
Since we're commanded to "receive, stand firm and hold to, and maintain" these traditions, how do you do that with respect to the traditions which were (at that time) oral?
The oral teaching of all the Apostles was spreading rapidly through the Levant, the eastern Mediterranean, Egypt/Ethiopia, the province of Asia (Anatolia) and western Asia generally, --- spreading more rapidly than the written Tradition (NT Scripture). It's this oral tradition and example (plus the OT) which was the very basis for the establishment of local congregations. They were ALWAYS founded on the basis of Oral Apostolic Tradition; only later (a few years, a generation later) was that supplemented with the whole written canon of the NT. Obviously. Local churches were being planted through those regions even before the last book of the NT was written ---
What you'd have to do is look and see what doctrines, practices, prayers/hymns, observances, ways of life, all those communities had in common.
That some doctrines and practices were common to all believers, on three continents, shows that these far-flung congregations had not devised themselves as a DIY religion, but had gotten ways f worship and ways of life from their Apostolic teachers. The new local churches heeded their preaching, passed on their teaching, imitated them and followed their example.
Excuse my dilation on this topic. It may all be obvious to already, but I wanted to throw in my two cents here.
Little-t tradition. can change. Big-T Apostolic Tradition is a "deposit": other than "spelling out" its own innate implications, it essentially remains the same.
For instance, compare the Apostles' Creed to the slightly later ones. The later ones explain and disambiguate the meaning of the very earliest Creed.
Jude 1:3
Beloved, although I made every effort to write you about the salvation we share, I felt it necessary to write and urge you to contend earnestly for the faith entrusted once for all to the saints.
He says he made every effort to write; he says it's also necessary to "contend earnestly" for the faith "entrusted" to them. Evidently it was not all entrusted by writing; it was also entrusted by word of mouth and by imitation/example.
St. John says repeatedly, that there was a LOT more that wasn't written down at that point, toward the end of his Gospel and his Epistles. And he was one of the last to put a portion of his teachings into writing.
Do you ever think about that? What was that 'lot more'? You're not supposed to be indifferent to it, or think it's not knowable. You've got to contend for it.