Stephen Westerholm movingly summarizes Augustine’s view on election and reprobation (Perspectives Old and New on Paul: The ‘Lutheran’ Paul and His Critics [Grand Rapids: Eerdmans, 2004], 19):

Augustine can offer no explanation why God chooses some and not others. Any explanation based on a difference in the human beings concerned would mean there was something in some human beings that God recognized and rewarded; their salvation would no longer depend on his grace alone. [n. 26: Note, however, Augustine’s earlier thinking in De div. quaest. 68.4.] If, on the other hand, the decision rests exclusively on God’s sovereign will, then it must remain impenetrable for human minds.

But Augustine does insist on the justice of what God does. Blame for human sin rests on humans alone—God has not compelled them to sin [n. 27: Texts that speak of God “hardening” sinners can only mean that God leaves them in their sinful ways; Augustine will not allow that God plays any active role in promoting sin (Ad Simpl. 1.2.15–16).]—and all merit condemnation. The condemned are justly condemned (Ench. 24.94); and since justice itself is a good thing, there is even goodness in their condemnation. Furthermore, that some are condemned reminds the forgiven of what their sins merit and of the magnitude of God’s mercy (De don. pers. 12.28). As one to whom many owe money can decide to cancel the debts of some and exact payment from others, and it would be presumptuous of the debtors either to think the decision should be theirs or to fault their creditor for exercising a creditor’s prerogative, so God is free to show mercy as he pleases without doing injustice to any (Ad Simpl. 1.2.16). If God displays his mercy in some and his justice in others, there is goodness in his justice and justice in his mercy (De don. pers. 12.28).