For an example of the just judgment of God. That is, that the persecutions and troubles you suffer in this world shew the justice of God in punishing men for their sins, even in this life, so that by these temporal pains you may be found worthy of a crown of eternal glory in the kingdom of God. (Witham)

The afflictions, which are here frequently the portion of the just, are sensible proofs of the rigour with which the Almighty will, at the day of final retribution, pour out his indignation on the wicked. For, if he is unwilling to let the just be free from all temporal punishment, (though he discharges their debt of the eternal) and if he continually exposes them to the derision, calumnies, and persecutions of the wicked, what have not the wicked to apprehend when he shall stretch forth his hand in vengeance? Or, as others explain it, God permits the good to be persecuted here, that one day he may treat the wicked according to the rigour of his justice. He permits them here to fill up the measure of their iniquities, that on the last day he may reward the long suffering of the one, and punish the infidelity of the other. In both the one and the other, the finger of God’s justice will clearly manifest itself. If the hopes of the good reached no farther than this life, they would be the most wretched of beings; for here, in general, they are more exposed than any to the injuries of the wicked. Nothing proves more clearly the necessity of a general judgment, than this his conduct to his most chosen servants. For it is impossible that, just as he is, he should permit patience and faith to go unrewarded, or wickedness and injustice unpunished. The Son of God has promised us heaven only on condition that we bear wrongs patiently. (Calmet)

Here again the apostle teaches the advantages of sufferings which the Thessalonians joyfully underwent, to be counted worthy of the kingdom of God, Kataxiothenai umas; and ver. 11, ibid. axiose. The apostle teaches here, that nothing defiled shall ever enter into the kingdom of heaven; and gives us to understand at the same time, that he will one day punish with extreme rigour the cruelty and impiety of persecutors. (Bible de Vence)