Dating 📜
One of the earliest known pieces of Christian writing is the Thessalonians epistle. It is usually dated between 50 and 51 CE. But it’s generally dated 50-60 CE. It is universally acknowledged to be a genuine Paul letter. Thessalonica was Macedonia’s capital and a significant seaport. It is believed that Paul wrote the letter to the Thessalonians from Corinth a few months after establishing a congregation there.

Interpolations

Burton Mack writes of 1 Thess. 2:14-16 in his Who Wrote the New Testament? (p. 113):
“The person who made this change was interested in directing Paul’s apocalyptic preachments against those who opposed the Christian mission and did so by inserting a small unit aimed specifically at the Jews who ‘killed Jesus’ and ‘drove us out,’ for which reason ‘God’s wrath has overtaken them at last.’ Nothing in all of Paul’s letters comes close to such a pronouncement (Pearson 1971). The idea seriously tarnishes the inclusive logic of the Christ myth, and it presupposes the logic of Mark’s passion narrative which, as we shall see, runs counter to that of the Christ myth. And since, according to this addition, it was the Jews upon whom God’s wrath had (already) fallen, the reference must surely be to the destruction of the temple in 70 C.E., an event that Paul did not live to see.”
Udo Schnelle comments on the same passage (The History and Theology, p. 48):
I Thess. 2.14-16 has often been regarded as a post-Pauline interpolation. The following arguments have been based on the content: (1) the contradiction between Romans 9-11 and 1 Thess. 2.14-16. (2) The references to what has happened to Jews as a model for a Gentile Christian church. (3) There were no extensive persecutions of Christians by Jews in Palestine prior to the first Jewish war. (4) The use of the concept of imitation in 1 Thessalonians 2.14 is singular. (5) The aorist εφτασεν (has overtaken) refers to the destruction of Jerusalem.

Schnelle maintains that these arguments are insufficient (op. cit., p. 48):
(1) The tension between 1 Thessalonians 2.14-16 and Romans 9-11 goes back to Paul himself. It is a problem that needs to be explained, not a problem to be set aside by interpolation hypotheses. (2) Paul’s ecclesiology presupposes a church of Jewish and Gentile Christians, so that Jewish Christians in Palestine can in fact serve as a model for Gentile Christians elsewhere. (3) Prior to 70 CE there were already conflicts between Jews and Christians in Palestine (cf. Luke 6.22). (4) The concept of imitation in 1 Thessalonians 2.14 is found already in 1 Thessalonians 1.6. (5) 1 Thessalonians 2.16c does not have the destruction of Jerusalem in view, but Paul sees in the hostile conduct of the Jews that the wrath of God has come to completion.
Raymond Brown mentions two additional reasons that the passage might be considered to be an interpolation. The first is that, “It constitutes a second Thanksgiving in the letter” (An Introduction, p. 463). The second is that, “The statement that the Jews ‘are the enemies of the whole human race’ resembles general Pagan polemic, scarcely characteristic of Paul.” Yet Brown goes on to mention arguments in favor of authenticity (op. cit., p. 463):
(a) All mss. contain it; (b) Paul speaks hostilely of ‘Jews’ as persecutors in II Cor 11:24, and he is not incapable of polemic hyperbole; (c) In Rom (2:5; 3:5-6; 4:15; 11:25) Paul speaks of the wrath of God against Jews, so that the hope of their ultimate salvation does not prevent portrayal of divine disfavor.

It is also sometimes suggested that 5:1-11 is “a post-Pauline insertion that has many features of Lucan language and theology that serves as an apologetic correction to the Pauline expectation of the parousia and thus already reflects the problem of the delay of the parousia” (Schnelle, p. 48).

Paul had made it abundantly clear in his teaching that Jesus would appear to those who were alive at the time. The Thessalonian church became concerned about the fate of those who had passed away prior to the Lord’s return as a result of this teaching. Would they share in the parousia’s joy? Paul writes to the Thessalonians to reassure them that those who had given up on Christ would benefit from the Lord’s return as well. They are instructed by Paul to join the living when the Lord returns, as the dead will first rise to life.

More analysis on the area of the interpolation (1 Thessalonians 2:13-16) 📜
“First of all, Paul and the other Jesus followers continue to identify as Jews. Another thing is that neither Pilate nor the Romans are mentioned! People weren’t crucified by the Jewish leadership, and they didn’t have the power to do so.”
Response: First of all, there is no rhetorical point in mentioning the Romans because the author is asking his Thessalonian readers to imitate the example of Christians in the Judean churches, who persevere amid conflict from their own countrymen (ἰδίων συμφυλετῶν). Also for the Christians in Judea to be persecuted by their own countrymen, this shows that the author’s polemic was an intra-Jewish one and not one that forgot that Paul and his followers were Jews (cf. 2 Corinthians 11:24-26 in which Paul says that he received 39 lashes five times “from the Jews” and was in danger from “my fellow Jews”). 1 Thessalonians 2:14-15 has been typically read as a blanket condemnation of Jews in general but several scholars point out that ὑπὸ τῶν Ἰουδαίων τῶν καὶ τὸν κύριον ἀποκτεινάντων is not necessarily non-restrictive (as the comma in most translations implies) and in fact the articular participle points to the expression being restrictive with a narrower scope (“the Jews that killed the Lord”). Articular participles tend to be restrictive in Paul and this is also the case in the prior verse which mentions “the churches that are in Judea” (τῶν ἐκκλησιῶν τῶν οὐσῶν ἐν τῇ Ἰουδαίᾳ) in 2:14 (cf. 1 Corinthians 1:2, 2 Corinthians 1:1). So the condemnation is directed at Jews (Judeans) who hinder the work of the church. What is however odd here is that “Paul” is silent about his own past complicity in persecution in Judea which he did not hesitate to mention elsewhere (Galatians 1:13, 23; 1 Corinthians 15:9, Philippians 3:6).
Even though crucifixion is a Roman form of capital punishment, this does not rule out the role of Jewish authorities in arresting and handing over Jesus to Pilate. Particularly if Jesus was causing disturbances in the Temple precincts as the “cleansing” story maintains, and if his predecessor John was executed by Herod Antipas (as Josephus relates) and if James bar-Zebedee was executed by Herod’s nephew Agrippa. Also it is noteworthy that Paul’s favorite verb σταυρόω is not used here (perhaps reflecting an awareness that the Jews did not perform crucifixion themselves) but ἀποκτείνω which shares its reference with the prophets of old, which is OT LXX vocabulary (cf. τοὺς προφήτας σου ἀπέκτειναν in 1 Kings 19:10, 14, Nehemiah 9:26).
Thirdly, why would anyone prevent the followers of Jesus from speaking with Gentiles? Why would they care or even be aware of that at such a young age?
Response: On the contrary, this was probably a major issue at the time (if the passage is original to 1 Thessalonians). With the letter written around 49 or 50 CE, there were two major events that occurred in those years: Claudius’ expulsion of the Jews from Rome and the Jerusalem council on the place of Gentiles in the Jesus movement. The much-discussed reference in Suetonius is often taken to mean that Christian proselytism created conflict with Jews in synagogues because Christians invited Gentiles to become full members of the community without requiring a bare minimum of Torah observance (e.g. kashrut, circumcision, sabbath observance). This led to disturbances and the need to have a meeting to sort out exactly what was to be required of Gentile proselytes. Another indication that ritual separation between Jews and Gentiles is in view in v. 16 can be found in the immediately preceding clause πᾶσιν ἀνθρώποις ἐναντίων, which invokes the Gentile charge of misanthropy that arose on account of ritual separation. Paul himself mentions the prospect of conflict with Jews in Judea in Romans 15:31: “Pray that I may be kept safe from the unbelievers in Judea”.

And what does it imply that God’s wrath has finally prevailed over them? That echoes what you might have said after the Temple was destroyed. For Paul to say such a thing in the year 51 AD, what was going on?
Response: The destruction of the Temple in 70 CE was the most significant event, but Judea suffered greatly from the 45s to 50s CE. First there was the starvation in Judea in 46-48 CE (Josephus, Relics 20.51-53, 101; cf. Acts 11:27–28, which conflates this famine with the one that occurred in Greece and Italy a few years later), during which many people starved to death. Then there was the catastrophe that occurred during the Passover celebration in Jerusalem in 49 CE, when 30,000 people were crushed to death in a stampede at the Temple (Josephus, Bellum Judicium 2.224-227). According to Josephus, “the festival turned into mourning for the nation as a whole, lamentation in every household.” Then there was the Jewish expulsion from Rome in 49 CE, which would have further exacerbated the chaos in Judea. Someone with a personal vendetta might have believed that these ongoing calamities were the result of God’s wrath.

More on 1 Thessalonians 2:13-16 as an interpolation 📜
https://www.jstor.org/stable/1508972
Most scholars have concluded this was never written by Paul. The arguments are many, and accumulate to a conclusive case:
Paul never blames the Jews for the death of Jesus elsewhere.

Paul never talks about God’s wrath as having come, but as coming only at the future judgment (see: Romans 2:5, 3:5-6, 4:15).

Paul teaches the Jews will be saved, not destroyed (see: Romans 11:25-28).

Paul was dead by the time the “wrath had come upon them to the uttermost” (the destruction of the Jewish nation and temple in 70 A.D.).
Most scholars agree it’s not written by Paul: Birger Pearson, “1 Thessalonians 2:13-16: A Deutero-Pauline Interpolation,” Harvard Theological Review 64 [1971]: 79-94; G. E. Okeke, “1 Thessalonians 2.13-16: The Fate of the Unbelieving Jews,” New Testament Studies 27 [1981]: 127-36.)
1 Thessalonians 2:13-16 is very unusual in several ways. Not in any of Paul’s 20,000 words, and dozens of discussions of the Jews, is anything like it. That immediately casts it into doubt. Paul blaming the Jews for the death of Jesus is simply unprecedented. Paul also never talks about the Jews as if he wasn’t one of them (see: Galatians 2:15; 1 Corinthians 9:20; Romans 9:1-5, 11:1; Philippians 3:4-5). And Paul acknowledged Jews as members of his own church, so he wouldn’t damn them as a group like this, and never does (see: 1 Corinthians 1:24, 12:13; 2 Corinthians 11:12; Romans 9:24, 10:12; on how this interpolation is undeniably–and uncharacteristically for Paul–Antisemitic).
Instead, Paul says things like… “Did God cast off his people? God forbid! For I also am a Jew, of the seed of Abraham, of the tribe of Benjamin” (Romans 11:1). “Are they Hebrews? So am I. Are they Israelites? So am I. Are they the seed of Abraham? So am I” (2 Corinthians 11:22). That Paul actually taught the Jews would be saved, not damned, is clear throughout his letters, for instance in Romans 11:25-28:
For I would not have you ignorant of this mystery, lest you be wise in your own conceits, that a hardening in part hath befallen Israel, until the fulness of the Gentiles be come in; and so all Israel shall be saved: even as it is written…and as touching the gospel, they are enemies for your sake: but as touching the election, they are beloved for the fathers’ sake.
Paul is writing to pagan converts (see verse 1:9) being persecuted by pagans, not by Jews (this is what he means in the authentic part of verse 2:14, highlighted above), so why would he suddenly break into a tirade against “the Jews” here? This makes no sense in context and violates the entire thread of his argument, that the Thessalonians are awesome for having withstood a pagan persecution.

The passage also says God’s wrath has come upon the Jews “to the uttermost” (literally “to the end” / “with finality”). Attempts to reinterpret this as not meaning “with finality” is simply trying to get a word to mean the exact opposite of what it actually means. More importantly, the remark unmistakably refers to something that affected the Jews in Judea (“For you became imitators of the churches of God which are in Judaea … for you also suffered the same things of your own countrymen as they did of the Jews who [killed Jesus and the prophets in Judea, and drove us out of Judea, etc.]…”), so the attempt to claim it refers to an earlier expulsion of Jews from Rome is a complete non-starter. That was a purely temporary and isolated event (and thus not by any stretch of the imagination “final”), and hardly anything one would call the wrath of God (unless you think God is really lame–as if the worst he could do to display his “wrath” is force some Jews living in pagan Rome to go back to the Holy Land), and in any case only affected Jews in Rome, not Jews in Judea (so how could God’s wrath have been visited on the Jews of Judea by punishing Jews in Rome?).