Paul reminded his readers how they had welcomed the gospel message to vindicate further his own ministry and to emphasize the importance of proclaiming this message. He did this so the Thessalonians would continue to herald it abroad as they had been doing. [source][source][source]
"This section of the letter begins with the second thanksgiving in a series of three ( 1 Thessalonians 1:2-5; 1 Thessalonians 2:13; 1 Thessalonians 3:9-13) that dominate the tone of the first three chapters." [1][source]