14 As regards the deeds of the ancients, both those which seem sinful to foolish and ignorant people, when they are not so, and those which really are sinful, we have already explained why they have been written, and how this rather adds to than impairs the dignity of Scripture. So, too, about the curse on him who hangs on a tree, and on him who raises not up seed in Israel, our reply has already been given in the proper place, when meeting Faustus' objections. And in reply to all objections whatsoever, whether we have already answered them separately, or whether they are contained in the remarks of Faustus which we are now considering, we appeal to our established principles, on which we maintain the authority of sacred Scripture.
The principle is this, that all things written in the books of the Old Testament are to be received with approval and admiration, as most true and most profitable to eternal life; and that those precepts which are no longer observed outwardly are to be understood as having been most suitable in those times, and are to be viewed as having been shadows of things to come, of which we may now perceive the fulfillments. Accordingly, whoever in those times neglected the observance of these symbolic precepts was righteously condemned to suffer the punishment required by the divine statute, as any one would be now if he were impiously to profane the sacraments of the New Testament, which differ from the old observances only as this time differs from that.
For as praise is due to the righteous men of old who refused not to die for the Old Testament sacraments, so it is due to the martyrs of the New Testament. And as a sick man should not find fault with the medical treatment, because one thing is prescribed today and another tomorrow, and what was at first required is afterwards forbidden, since the method of cure depends on this; so the human race, sick and sore as it is from Adam to the end of the world, as long as the corrupted body weighs down the mind, should not find fault with the divine prescriptions, if sometimes the same observances are enjoined, and sometimes an old observance is exchanged for one of a different kind; especially as there was a promise of a change in the appointments.
Source: Reply to Faustus the Manichaean (New Advent)