This is an obvious sign of the righteous judgment of God, to the end that you may be counted worthy of the Kingdom of God, for which you also suffer.
"Jesus encouraged his disciples to rejoice when they were persecuted for his sake because, he said, ’your reward is great in heaven’ (Matthew 5:11-12 par. Luke 6:22-23). This note recurs again and again throughout the NT." [Note: F. F. Bruce, 1 and 2 Thessalonians, p. 154.] 