Augustine was not a legal positivist, which is to say, he knew that human laws and customs could not be ultimate. If they are out of accord with a higher law (viz. natural law or divine law), they are not truly binding because not truly just. The opposite of this position, of course, is the position of Thrasymachus in Plato’s Republic, which is to say, the position that might makes right. This position does not destroy human custom–it only holds it to account. Human custom, Augustine well knows, will vary depending upon place and time, but the higher law which it instantiates–the higher law to which it must be subservient–does not vary depending upon time and place. The idea, then, is one of unchanging principle adapted to changing circumstances.
There are various places in which Augustine elucidates what has been the standard classical and Christian view of justice from antiquity until the very recent past. One place in which he does so is Confessions 3.7.13, 8.15.
13. Nor had I knowledge of that true inner righteousness, which does not judge according to custom, but out of the most perfect law of GodAlmighty, by which the manners of places and times were adapted to those places and times— being itself the while the same always and everywhere, not one thing in one place, and another in another; according to which Abraham, and Isaac, and Jacob, and Moses, and David, and all those commended by the mouth of God were righteous, Hebrews 11:8-40 but were judged unrighteous by foolish men, judging out of man’s judgment, 1 Corinthians 4:3 and gauging by the petty standard of their own manners the manners of the whole human race. Like as if in an armoury, one knowing not what were adapted to the several members should put greaves on his head, or boot himself with a helmet, and then complain because they would not fit. Or as if, on some day when in the afternoon business was forbidden, one were to fume at not being allowed to sell as it was lawful to him in the forenoon. Or when in some house he sees a servant take something in his hand which the butler is not permitted to touch, or something done behind a stable which would be prohibited in the dining-room, and should be indignant that in one house, and one family, the same thing is not distributed everywhere to all. Such are they who cannot endure to hear something to have been lawful for righteous men in former times which is not so now; or that God, for certain temporal reasons, commanded them one thing, and these another, but both obeying the same righteousness; though they see, in one man, one day, and one house, different things to be fit for different members, and a thing which was formerly lawful after a time unlawful— that permitted or commanded in one corner, which done in another is justly prohibited and punished. Is justice, then, various and changeable? Nay, but the times over which she presides are not all alike, because they are times. But men, whose days upon the earth are few, Job 14:1 because by their own perception they cannot harmonize the causes of former ages and other nations, of which they had no experience, with these of which they have experience, though in one and the same body, day, or family, they can readily see what is suitable for each member, season, part, and person— to the one they take exception, to the other they submit.
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15. Can it at any time or place be an unrighteous thing for a man to love God with all his heart, with all his soul, and with all his mind, and his neighbour as himself? Therefore those offenses which be contrary to nature are everywhere and at all times to be held in detestation and punished; such were those of the Sodomites, which should all nations commit, they should all be held guilty of the same crime by the divine law, which has not so made men that they should in that way abuse one another. For even that fellowship which should be between God and us is violated, when that same nature of which He is author is polluted by the perversity of lust. But those offenses which are contrary to the customs of men are to be avoided according to the customs severally prevailing; so that an agreement made, and confirmed by custom or law of any city or nation, may not be violated at the lawless pleasure of any, whether citizen or stranger. For any part which is not consistent with its whole is unseemly. But when God commands anything contrary to the customs or compacts of any nation to be done, though it were never done by them before, it is to be done; and if intermitted it is to be restored, and, if never established, to be established. For if it be lawful for a king, in the state over which he reigns, to command that which neither he himself nor any one before him had commanded, and to obey him cannot be held to be inimical to the public interest,— nay, it were so if he were not obeyed (for obedience to princes is a general compact of human society),— how much more, then, ought we unhesitatingly to obey God, the Governor of all His creatures! For as among the authorities of human society the greater authority is obeyed before the lesser, so must God above all.
