Where does the historical evidence come from?
1 Thessalonians, which is usually reckoned to be the oldest book in the New Testament, the concensus dating it from AD 50 to 52. The dating is estimated from internal evidence: for one the letter starts off with a greeting from “
Paul, Sylvanus, and Timothy,” ‘Sylvanus’ being believed to be Silas, who accompanied Paul on his second missionary journey (ca. AD 49-51; but see below). Timothy also joined the second missionary journey early on (Acts 16:1); however, Silas apparently did not accompany Paul on the third missionary journey, and this trio are not mentioned together again. Therefore, the greeting appears to come some time during their second missionary journey, which would be after they visited Thessalonica and established a church there (Acts 17:1-4). The letter is generally believed to have been composed by Paul somewhere during his first visit to Corinth (Acts 18:1-18), just shortly after Claudius issued an edict evicting Jews from Rome (v. 2; cf. Suetonius,
Claudius 25), which the 5th century author Paulus Orosius (
Historia 7.6.15f) has dated to Claudius’ fifth regnal year, i.e. around AD 49.
That being said, Orosius is contradicted by the 3rd century historian
Cassius Dio (
Roman History 60.6.6), who dates the event to Claudius’ first year as emperor (AD 41). As a result the experts are divided over when this expulsion took place - some argue for an earlier date while others for a later one. Supposing that the earlier date is correct, this would affect the chronology of Paul’s life, not only the time of his arrival in Corinth and his second missionary journey, but also the dating of 1 Thessalonians: it could have well been written ten years earlier.
Another clue usually given is the theme of the letter. Elsewhere Paul seems to focus on themes such as justification by faith or questions of Jewish-gentile relations: here the emphasis on such topics is still not present, which could suggest that Paul did not formally think of them yet. Many scholars thus see this as an indication that this letter was written before the Epistle to the Galatians (AD 50-60), where Paul formed and identified his positions on these matters.