1 Corinthians 11:2: “. . . keep the ordinances, as I delivered {them} to you.”
2 Thessalonians 2:15: “. . . hold the traditions which ye have been taught, whether by word, or our epistle.”
2 Thessalonians 3:6: “Now we command you, brethren, in the name of our Lord Jesus Christ, that ye withdraw yourselves from every brother that walketh disorderly, and not after the tradition which he received of us.”
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OK, now, tell us exactly what oral tradition it was to which Paul was referring and how anyone is to know that it was faithfully handed down over 2,000 years unchanged.
How is it verified both to be from Paul and as he stated it?
It’s right in front of your eyes. Paul refers to oral teaching he did or intends to do. So does Luke. So does John. So does Jude. So does Peter. ALL refer to oral teaching from the apostles or those sent by them as authoritative and binding. If you dismiss apostolic teaching just because it’s unwritten, you dismiss the written scripture also. They are indissolubly linked, a single body of teaching.
We know the teaching was faithfully handed down because we have the promise of Christ that the Church he founded and entrusted to his apostles would teach faithfully and with authority.
A better question is this: why do Protestants believe the Bible is trustworthy? Seriously — why? How do they do that without relying on the authority and tradition they claim to reject? How do they live with this level of denial and incoherence?