Heidegger's Bible Handbook: 1 Thessalonians: Occasion of the Epistle

2. The occasion of the writing of the Epistle furnished by the narration of Timothy concerning them.

Therefore, Timothy, having returned from Thessalonica to Paul, supplied the occasion for the writing of this Epistle, εὐαγγελισάμενος τὴν πίστιν καὶ ἀγάπην, bringing good tidings of their faith and love, 1 Thessalonians 3:6. And he, being not a little cheered by the hearing of this announcement, undertook to congratulate them for their constancy, and further to confirm their faith.