"So then, brethren, stand firm and hold to the traditions which you were taught by us, either by word of mouth or by letter."
Protestants believe that Christians are to follow the Scriptures alone as their sole source of Christian truth (sola Scriptura). But then why does Saint Paul tell us to follow both the Scriptures and the oral word? Isn't Paul adding something else to follow in addition to the Bible? Yes he is, because the doctrine of sola Scriptura is an erroneous doctrine.
Saint Paul is saying that obeying the written tradition (the Scriptures) is not enough. We must also obey the oral tradition. This is the body of teaching that Christ gave the apostles that was not written down (if it were, Saint John says that "even the world itself could not contain the books that would be written." John 21:25). In other words, it's everything else the Church teaches on faith and morals. We can be thankful for the oral apostolic traditions which have definitively taught us about the Blessed Trinity, the two natures of Christ (human and divine), the union of those natures (hypostatic union), the Filioque (the Spirit proceeds from the Father and the Son), and the canon of Scripture (what books belong in the Bible and what books do not). All of these teachings, and many, many more, are not explicitly taught in the Bible, yet are generally believed by all Christians. To learn more about the oral apostolic tradition, buy a copy of the Catechism of the Catholic Church.
Because 2 Thessalonians 2:15 is so troubling to the sola Scriptura position, Protestants often argue that the oral tradition Paul is referring to had to come from the mouths of the apostles. Their argument further goes that, since all the apostles are deceased, we no longer have to follow oral tradition. This argument, however, cannot be proven from Scripture (which should be possible if sola Scriptura were true) and, in fact, is contrary to Scripture. See for example, 2 Timothy 2:2 where Paul (1st generation) instructs Timothy (2nd generation) to teach others the faith (3rd generation) who will be able to teach others also (4th generation). Such an argument is also inconsistent with the very meaning of tradition (in Greek, "paradosis") which means "to hand on" from one generation to the next.
Moreover, the Protestant argument is also refuted by the way in which the Church selected the Bible canon. While the last apostle John died around 100 A.D., the Bible was not finally compiled until 397 A.D. The Church was thus required to rely upon the oral apostolic tradition during this 300 year period in order to determine which letters were inspired and which letters were not. The tradition they depended on, of course, did not come from the mouths of the apostles (they were deceased), but from their successors. (There is also no reason to conclude that the Church should listen to the fourth, fifth or sixth generation of apostolic successors, but not to later successors such as those of our day).
We should also note that the apostolic traditions Paul is commanding us to follow in 2 Thessalonians 2:15 are not the same as the Pharisaical traditions that Jesus condemned in Matthew 15:3 and Mark 7:9. The traditions Jesus condemned dealt with the Old Testament ceremonial rituals and other acts that contravened the New Testament Gospel. So there are certain human traditions that, if contrary to the Gospel, we must reject, and oral apostolic tradition, as Paul commands, which we must accept.
The only other argument the Protestant can make is that, once the Bible was compiled, all oral apostolic tradition was committed to the Scriptures. As a result, the requirement to follow oral tradition ceased. But this they cannot prove from the Bible. There is nothing in the Scriptures that commands us to follow oral tradition until the Bible is compiled, and then to follow the Bible alone (the word "Bible" is not even in the Bible). In fact, Jesus never even commanded any of His apostles to write anything down. They were only charged to "preach the Gospel to all creation." Matthew 28:19. Because the Scriptures are the living word of God which is the same yesterday, today and forever (cf. Hebrews 13:10), and there is no verse in Scripture that repudiates Paul's instruction in 2 Thessalonians 2:15, we must also obey the oral tradition of the Church as Paul commanded, or we are not being faithful to the Scriptures.
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