2 Thessalonians 1:5

"Which is a manifest token of the righteous judgment of God." See how he gathers comfort for them. He had said, We give thanks to God, he had said, We glory among men: these things indeed are honorable. But that which he most seeks for, who is in suffering, is, deliverance from evils, and vengeance upon those who are evil entreating them. For when the soul is weak, it most seeks for these things, for the philosophic soul does not even seek these things. Why then does he say, "a token of the righteous Judgment of God"? Here he has glanced at the retribution on either side, both of those who do the ill, and of those who suffer it, as if he had said, that the justice of God may be shown when He crowns you indeed, but punishes them. At the same time also he comforts them, showing that from their own labors and toils they are crowned, and according to the proportion of righteousness. But he puts their part first. For although a person even vehemently desires revenge, yet he first longs for reward. For this reason he says, "That ye may be counted worthy of the kingdom of God, for which ye also suffer." This then does not come to pass from the circumstance that those who injure them are more powerful than they, but because it is so that they must enter into the kingdom. "For through many tribulations," he says, "we must enter into the kingdom of God."

Lastly, as regards Faustus' crafty insinuation, that the Old Testament misrepresents God as threatening to come with a sword which will spare neither the righteous nor the wicked, if the words were explained to the Pagan, he would perhaps disagree neither with the Old Testament nor with the New; and he might see the beauty of the parable in the Gospel, which people who pretend to be Christians either misunderstand from their blindness, or reject from their perversity. The great husbandman of the vine uses his pruning-hook differently in the fruitful and in the unfruitful branches; yet he spares neither good nor bad, pruning one and cutting off the other. There is no man so just as not to require to be tried by affliction to advance, or to establish, or to prove his virtue. Do the Manichaeans not reckon Paul as righteous, who, while confessing humbly and honestly his past sins, still gives thanks for being justified by faith in Jesus Christ? Was Paul then spared by Him whom fools misunderstand, when He says, "I will spare neither the righteous nor the sinner"? Hear the apostle himself: "Lest I should be exalted above measure by the abundance of the revelation, there was given me a thorn in the flesh, a messenger of Satan to buffet me. For this I besought the Lord thrice, that He would remove it from me; and He said unto me, My grace is sufficient for thee: for strength is perfected in weakness." … Paul also, besides recording his own experience, says that the afflictions and persecutions of the righteous exhibit the judgment of God. This truth is set forth at length by the Apostle Peter in the passage already quoted, where he says: "It is time that judgment should begin at the house of the Lord. And if it first begin at us, what shall the end be of those that believe not the gospel of God? […]

He calls the judgment of God "just" because then God will judge the world equally.

And the saints endure these tribulations for the sake of two things. One is to strike fear into the wicked: for if God does not spare the good in this world, how will he spare the wicked in the next? "And if it begins with …

so that you may be held worthy of the kingdom of God, for which you also suffer The persecutions and tribulation did not make them worthy, but rather the grace of God that bestows to them the power of enduring those things patiently.