20 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 Manichæans 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 you: for strength is perfected in weakness." Here a just man is not spared that his strength might be perfected in weakness by Him who had given him an angel of Satan to buffet him.
If you say that the devil gave this angel, it follows that the devil sought to prevent Paul's being exalted above measure by the abundance of the revelation, and to perfect his strength. This is impossible. Therefore He who gave up this righteous man to be buffeted by the messenger of Satan, is the same as He who, through Paul, gave up to Satan himself the wicked persons of whom Paul says: "I have delivered them to Satan, that they may learn not to blaspheme." Do you see now how the Most High spares neither the righteous nor the wicked?
Or is it the sword that frightens you? For to be buffeted is not so bad as to be put to death. But did not the thousands of martyrs suffer death in various forms? And could their persecutors have had this power against them except it had been given them by God, who thus spared neither the righteous nor the wicked? For the Lord Himself, the chief martyr, says expressly to Pilate: "You could have no power at all against me, except it were given you from above." 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? And if the righteous scarcely are saved, where shall the ungodly and the sinner appear?" Peter also explains how the wicked are not spared, for they are branches broken off to be burnt; while the righteous are not spared, because their purification is to be brought to perfection. He ascribes these things to the will of Him who says in the Old Testament, I will spare neither the righteous nor the wicked; for he says: "It is better, if the will of the Spirit of God be so, that we suffer for well-doing than for evil-doing." So, when by the will of the Spirit of God men suffer for well-doing, the righteous are not spared; when they suffer for evil-doing, the wicked are not spared.
In both cases it is according to the will of Him who says: I will spare neither the righteous nor the wicked; correcting the one as a son, and punishing the other as a transgressor.
Source: Reply to Faustus the Manichaean (New Advent)