C. S. Lewis shows the influence of St. Augustine on Milton's theology of the fall in chapter 10 of A Preface to Paradise Lost. Some of the specific doctrines of this theology are as follows:

1. God created all things without exception good, and because they are good, 'No Nature (i.e. no positive reality) is bad and the word Bad denotes merely privation of good.' (De Civ. Dei, xi, 21, 22). ...

2. What we call bad things are good things perverted (De Civ. Dei, xiv, 11). ...

3. From this doctrine of good and evil it follows (a) That good can exist without evil, as in Milton's Heaven and Paradise, but not evil without good (De Civ. Dei, xiv, 11). (b) That good and bad angels have the same Nature, happy when it adheres to God and miserable when it adheres to itself (ibid., xii, 1). ...

4. Though God has made all creatures good He foreknows that some will voluntarily make themselves bad (De Civ. Dei, xiv, 11) and also foreknows the good use which he will then make of their badness (ibid.). For as He shows His benevolence in creating good Natures, He shows His justice in exploiting evil wills. . . . All this is repeatedly shown at work in the poem. God sees Satan coming to pervert man; 'and shall pervert,' He observes (III, 92). He knows that Sin and Death 'impute folly' to Him for allowing them so easily to enter the universe, but Sin and Death do not know that God 'called and drew them thither, His hell-hounds to lick up the draff and filth' (X, 620 et seq.). . . . The same doctrine is enforced in Book I when Satan lifts his head from the burning lake by 'high permission of all-ruling Heaven' (I, 212). As the angels point out, whoever tries to rebel against God produces the result opposite to his intention (VII, 613). At the end of the poem Adam is astonished at the power 'that all this good of evil shall produce' (XII, 470). This is the exact reverse of the programme Satan had envisaged in Book I, when he hoped, if God attempted any good through him, to 'pervert that end' (164); instead, he is allowed to do all the evil he wants and finds that he has produced good. Those who will not be God's sons become His tools.