Augustine's first move is to deny that shared suffering implies shared guilt. When the barbarians sacked Rome, good people and bad people suffered alike — they were robbed, violated, killed without discrimination. This seems to tell against providence. But Augustine insists the likeness of the suffering conceals a profound difference in what the suffering does. Affliction is not an index of guilt; it is a test, a purification, or a punishment, depending on what the sufferer is.
Augustine reaches for a series of images to make this concrete. The same fire that refines gold blackens straw. The same flail that beats out chaff cleans the wheat. The same press that produces wine from healthy grapes produces vinegar from sour ones. The instrument is the same; what it does depends entirely on what it works upon. Adversity is the instrument of providence — and providence's effects are as different as the characters of those who endure it.
Augustine does not shy away from the difficulty: if good people suffer alongside the wicked, does that not make God unjust? His answer is that God does not currently distribute punishment and reward with complete exactness — and deliberately so. If every sin were immediately punished, there would be no room for patience or growth; people would serve God out of fear of immediate consequences, not out of love. If every virtue were immediately rewarded, the same. The present unequal distribution of suffering is itself a form of mercy: it leaves space for both repentance and growth.
The analysis of suffering under the sack of Rome runs through most of Book I. Augustine's fire and gold image is among the most rhetorically powerful passages in the City of God, and his framing of adversity as revealing rather than creating character was enormously influential on medieval Christian ethics.