The explanatory for
(Greek: γάρ/gar) at the beginning of this sentence indicates that this verse provides background material which explains and strengthens the assertions of 1 Thessalonians 2:13.1 To state this more explicitly, the Thessalonians' willingness to suffer for the gospel gave Paul confidence that they had indeed accepted his message as the Word of God, thus strengthening the grounds for thanksgiving. In this way Paul affirms the believers in their faith.
14 For you, brothers, became imitators of the churches of God in Christ Jesus that are in Judea. For you suffered the same things from your own countrymen as they did from the Jews,