In Book 3, Augustine approaches the subject of how to interpret the literal and the figurative and addresses some issues regarding the customs and morality of the Old Testament. Augustine stresses the difficulties in dealing with figurative language, warning his readers that, “we must beware of taking a figurative expression literally” (216), and vice versa. The general solution Augustine offers is as follows: “[W]hatever there is in the word of God that cannot, when taken literally, be referred either to purity of life or soundness of doctrine, you may set down as figurative” (226). Augustine again emphasizes the ultimate end of scripture is to urge believers in the twofold love of their neighbor and the love of God.

In speaking of the Old Testament, Augustine admits that many of the customs depicted in the Old Testament seem at odds with Christian morality, such as the practice of polygamy by the Old Testament kings and patriarchs (253). His solution is to argue that things that were done at that time were time out of necessity and in a spirit of obedience to God and that such customs are now neither necessary nor acceptable (253).