It is fair to point out that the Thessalonians were at a disadvantage compared to Christians today. They did not have all the written books of New Testament Scripture. Paul wrote both of his epistles to Thessalonica very early in the New Testament era—about A.D. 51. The two letters were probably written only a few months apart and are among the very earliest of all the New Testament writings.
The Thessalonians’ primary source of authoritative gospel truth was Paul’s teaching. As an apostle, Paul taught with absolute authority. When he taught them, his message was the Word of God, and he commended them for recognizing that: “For this reason we also constantly thank God that when you received the word of God which you heard from us, you accepted it not as the word of men, but for what it really is, the word of God, which also performs its work in you who believe” (1 Thessalonians 2:13). Elsewhere he said that the commandments he gave them were “by the authority of the Lord Jesus” (1 Thessalonians 4:2).
The substance of what he taught them represented the same body of truth that is available to us in the New Testament. How do we know? Paul himself said so. Even as he was recording his inspired epistle to them, he reminded them, “Do you not remember that while I was still with you, I was telling you these things?” (2 Thessalonians 2:5). The written Word simply confirmed and recorded for all time the authoritative truth he had already taught them in person. These epistles were a written reminder of what they had already heard from Paul’s own mouth (1 Thessalonians 4:2).
Second Thessalonians 2:15 confirms this: “Stand firm and hold to the traditions which you were taught, whether by word of mouth or by letter from us.” There he declares, first of all, that his epistles to them are authoritative, inspired truth. This verse is a clear statement that Paul himself regarded these epistles as inspired Scripture.
But notice also that this verse joins the apostolic “traditions” with the written Word of God. The “traditions” necessary for Christians to be discerning are recorded for all ages in the text of Scripture. Those who claim that apostolic tradition is other truth in addition to Scripture often attempt to use this verse for support. Note, however, that Paul is not saying that “the traditions [they] were taught” are different from the written Scriptures. Rather, he links the two, affirming that the written Word of God is the only permanent and authoritative record of the apostolic tradition. He is specifically suggesting that the Thessalonians should not trust “word of mouth” or letters pretending to be from apostolic sources. Only what they had heard firsthand from Paul’s own lips or read in authentic letters from him were they to treat as authoritative, divine truth. That is why Paul usually signed his epistles “with [his] own hand” (1 Corinthians 16:21; Galatians 6:11; Colossians 4:18; 2 Thessalonians 3:17; Philemon 19).
With this in mind, 2 Thessalonians 2:15 cannot be used to support the claim that extrabiblical, spiritually binding “apostolic tradition” is passed down verbally through popes and bishops. Paul’s whole point was that the Thessalonians should treat as authoritative only what they had heard from his own mouth or received from his own pen. That body of truth—the Word of God—was to be the measuring stick they used to examine all things. Two other verses confirm this. In 2 Thessalonians 3:6 Paul writes, “Now we command you, brethren, in the name of our Lord Jesus Christ, that you keep away from every brother who leads an unruly life and not according to the tradition which you received from us.” And in verse 14 he adds, “If anyone does not obey our instruction in this letter, take special note of that person and do not associate with him, so that he will be put to shame.”
Therefore, Paul is affirming that the Bible is the only reliable criterion by which believers in this age can evaluate messages claiming to be truth from God.
The testing of truth that Paul calls for is not merely an academic exercise. It demands an active response.