Everyone agrees that Paul wrote a letter to the Christians at Thessalonica - among those in favour, the author of the New Testament's second letter thereto. This blog has been assuming that there was no real second letter. This derives from Bart Ehrman and lately Richard Carrier. Lately it occurs to me that the NT doesn't contain all of the Pauline corpus, forged or not. "Hebrews", "3 Corinthians", "Laodiceans" are famous; "2 Colossians" perhaps less-so. But also the Corinthians got a "lachrymose letter" after the first one; scholars still argue over that one's content, but all agree it was before or during canon 2 Cor.
So let us take our Second Thessalonians seriously - as Third Thessalonians.
As "3 Thessalonians", I for now set aside its authorship. I aim for what this missive was trying to refute, which was intimately involved with 1 Thess. "3 Thess", whether or not an authorial conceit, presented the Thessalonica Christendom as the recent recipients of a Pauline correspondence. Some say they can extract the Lachrymose content from what 2 Corinthians alludes. This poast will extract Deutero-Thessalonians from the allusions in what I'm (for now) calling "3 Thess".
What is refuted in "3 Thess" is not 1 Thess. "3 Thess" takes pains to parallel 1 Thess, as a common ground with the audience... and with Deutero-Thessalonians.
Deutero-Thessalonians stood in between. Its author was a Pauline, like the author of "Hebrews" and indeed like that of "1 Clement". Our man adhered closer than they to Paul's belief that the End was nigh. To repeat, 1 Thess was a shared text; although yet without the antiJewish interpolation, given that "3 Thess" won't address that pericope either. Deutero-Thessalonians also paralleled Philippians 4:5 that the Lord be at hand. Or even Romans 13:11-12.
Now: I doubt "3 Thess" would have attacked Philippians and I don't think anyone is claiming it had a problem with Romans, either. Deutero-Thessalonians had to stand out from these other works. One way it could so have done, would be to make claims about specific events of the Last Day. This is exactly what "3 Thess" will do: for that day will not come unless the rebellion comes first and the lawless one is revealed, the one destined for destruction
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Deutero-Thessalonians may have named an evil leader, termed "godless" and/or "christless". He would have gone to "destruction" as to herald the Day which Paul had promised. The laws of Rome no longer mattered any more than did the laws of the Jews. This all would be in parallel with the Revelation and 1 John, on "antichrists" - but stronger. One can ponder several Roman emperors who might fit this bill; over AD 68-9 Rome speedran four of them, as gamers might put it.
In Paul's own time, Tiberius had presided over the death of Jesus (and others), and Gaius Caligula was... Gaius Caligula. Claudius had enemies too.
The spiciest take of all might be that Paul authentically wrote Deutero-Thessalonians as his last testament on the coming apocalypse, from his prison in Spain. When he was executed and the apocalypse did not happen, his successors had to scribble out 3 Thess - now, "2 Thessalonians" - to stem the riot. The implication is that the rebels might be, themselves, breaking the restraint which is placed upon the antichrist. The eschaton is not to be immanentised.