Paul wrote this letter to the church in Thessalonica. He probably wrote in AD 49–51 from Corinth during his second missionary journey (Acts 18:1–18). 

From Bibles.net: Remember that the ultimate author of every book of the Bible is the Holy Spirit (2 Peter 1:21). He has written this book to equip you for life, to help you know the true God, and to give you hope (2 Timothy 3:16; Romans 15:4). The Holy Spirit wrote 1 Thessalonians for your good and to lead you into joy.

As we have already seen, Paul, Silas [as he is called in Acts, although the Greek here has the Latin form ‘Silvanus’] and Timothy were the missionary team who evangelized Thessalonica. It is natural, therefore, for Paul to associate Silas and Timothy with him in both his letters to the Thessalonians. This does not necessarily mean that they shared in composing them; it is more likely to have been a courteous gesture, since Silas and Timothy were so well known in the Thessalonian church, together with a general indication that they were in agreement with what Paul wrote. 

—John R. W. Stott 

Source: John. R. W. Stott in his book, The Message of 1 & 2 Thessalonians: Preparing for the Coming King, which is part of the series The Bible Speaks Today, published by IVP Academic.

Thessalonica was the capital of Macedonia, a Roman province in northern Greece. Boasting a population of more than 100,000, the city was a powerful commercial center in the Greco-Roman world. It was strategically located on the coast of the Aegean Sea at a key juncture along the Via Egnatia (a major Roman east-west highway). The city, therefore, attracted a diverse array of people and philosophies. This cosmopolitan makeup shaped its religious climate as well. While primarily polytheistic,[1] Thessalonica included a sizable number of monotheistic Jews.

In Acts 17:1–11, Luke recounts Paul’s visit to the city. He entered the local synagogue and on three consecutive Sabbaths “reasoned with them from the Scriptures” and proclaimed Jesus as the Christ (Acts 17:2–3 ESV). Some of the Thessalonians “were persuaded and joined Paul and Silas, as did a great many of the devout Greeks and not a few of the leading women” (Acts 17:4 ESV). Nevertheless, a band of jealous Jews “formed a mob, set the city in an uproar, and attacked the house of Jason, seeking to bring them out to the crowd” (Acts 17:5 ESV). Unable to find Paul, Silas, or Timothy, they dragged Jason and some others before the authorities and charged them with sedition: “These men who have turned the world upside down have come here also, and Jason has received them, and they are all acting against the decrees of Caesar, saying that there is another king, Jesus” (Acts 17:6–7 ESV). Narrowly escaping by night, Paul and his associates journeyed west to Berea, where, Luke notes, the Jews were “more noble than those in Thessalonica” (Acts 17:11 ESV). However, on learning that Paul was in Berea, some of the Thessalonian Jews “came there too, agitating and stirring up the crowds” (Acts 17:13 ESV). Paul again escaped, sailing south to Athens (Acts 17:16–33).

[1] Polytheism – The belief in or worship of multiple (poly) gods (theism). The Thessalonians served and revered various Greco-Roman deities, including Aphrodite, Demeter, Dionysus, and Zeus. Idolatry was rampant in the city (1 Thessalonians 1:9).

—Matt Smethurst

Source: Content taken from 1–2 Thessalonians: A 12-Week Study © 2017 by Matt Smethurst. All rights reserved. Used by permission of Crossway Books, a publishing ministry of Good News Publishers.

Paul has received a report from Timothy about the Thessalonian church. Paul writes to them to restore their hope, which has been tested by unexpected deaths in the church. He reassures them that both the dead and the living believers will be safe at the second coming (1 Thessalonians 4:13–5:11). In addition, Paul wants (1) to stress the authenticity of himself, Silas, and Timothy as preachers of the gospel (1 Thessalonians 1:5; 2:1–12; 2:17–3:10); (2) to teach them that persecution is normal for Christians (1 Thessalonians 3:3–4); and (3) to challenge them to take responsibility for earning their own living (1 Thessalonians 4:9–12). 

The Setting of 1 Thessalonians 

c. AD 49-51 

Paul wrote 1 Thessalonians from Corinth near the end of his second missionary journey. Paul and his companions had established the church in Thessalonica but were forced to leave by opponents of the gospel. Later, Paul sent Timothy back to Thessalonica to check on the church there, and Timothy’s report led Paul to write this letter. Thessalonica enjoyed privileged status as the capital of Macedonia and was located on a natural harbor along the busy east-west Egnatian Way.