Paul wrote to them in 1 Thessalonians and 2 Thessalonians.
History
Paul preached to them in the boldness in his God after suffering shameful treatment at Philippi. (1 Thessalonians 2:2)
They received the gospel in such power that they became famous in Macedonia, Achaia, and everywhere for their repentance from idolatry, their service to the living and true God, and their waiting for the Son to return from heaven.
Character
When Paul preached to them, they received the gospel in word, in power, and in the Holy Spirit with full conviction.
They became famous for their conversion, and Thessalonica being at a harbor and an intersection of trade routes, word spread everywhere. They imitated Paul, Silvanus, and Timothy's missionary band as they imitated the Lord and became an example in Macedonia and Achaia. Matthew Henry likens them to typoi—stamps, or instruments to make impression with.
(1 Thessalonians 1, Matthew Henry Bible Commentary (complete) - 1 Thessalonians 1 Bible Commentary @ https://www.christianity.com/bible/commentary/matthew-henry-complete/1-thessalonians/1)