This first letter speaks frequently about the persecution that the Thessalonian believers suffered ( 1 Thessalonians 1:6; 1 Thessalonians 2:14–16; 1 Thessalonians 3:3–5). Paul had been deeply concerned that they would buckle under the pressure and forsake the Lord (1 Thessalonians 3:5). Yet when Timothy returned from Thessalonica he brought the good news that the new converts were standing firm in the Lord (1 Thessalonians 3:6–8); they continued to believe in him, love him, serve him, even though it meant rejection and suffering. This is the essence of the endurance that Paul mentions here in the introductory thanksgiving: it is the capacity to continue to bear up under difficult circumstances.
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3 remembering before our God and Father your work of faith and labor of love and steadfastness of hope in our Lord Jesus Christ.