The Two Cities and Political Theology: I would argue that the City of God is the best work Augustine ever penned, despite its, at time, rambling, ad hoc, format. While the Two Cities model he offers has a broad conceptual range, it remains one of the most Biblically grounded formulations of the Church in a larger world. Sadly, one form of this was revealed when Augustine, not inconsistently, called for the state to suppress the Donatists. This was certainly one of the most shameful episodes of Augustine's life. However, this is so because City of God promises something better than this. Radical Augustinian not only recognizes temporal kingdoms as penultimate, but also pessimistically sees their foundation in the reign of the Devil. While Christians may safely live and function within the Roman world, its foundation remains a murdered brother (i.e. Romulus killing Remus). In this way, one can see the foundation of all Cities of Man as recapitulating Cain's sin. The diverse churches, founded in Jesus Christ, are representations of the City of God, one whose king shed His own blood to save His many brethren. Augustine's work remains absolutely crucial for Chrisitans faithfully assessing the world around them.
Sex, Sexuality, and Concupiscense: I still stand by my mostly negative assessment of Augustine on this. While he was a competent explore of man's psyche, and he was rather adept at unpacking desire, he never solved this problem. Despite apologies for his superior view over contemporaries, Augustine injected an essential sinfulness into all postlapsarian sexual encounters. This radical pessimism rewrites most of the Scriptures more benign sense of sex within the confines of proper order. Western theology has suffered for a long time under the weight of Augustine's unresolved tensions. While Luther represented a forceful rejection of Augustine, this was still mired in anthropological pessimism. Since, for Luther, all Human works, always, remained tainted by mortal sin, openly engaging in marital concupiscence was a means to flaunt the Devil's accusations. Ironically, on account of their reputation, English Puritans recovered a more robust view of Human sexuality within the bonds of matrimony. In today's age, full of warped desires and hyper-eroticisation, Augustine's complex understanding of desire may be needed, but his teaching about marriage and sex needs to be finally put to rest as defunct and blind.
Predestination: Augusitne engages in a bit of mythologizing about the elect replacing fallen angels in Heaven as a reason for God's saving grace. This is goofy. However, he is basically right. I think Augusitnian predestination needs to be reformatted along figural lines, and not through a trembling reflection on God's inscrutable will. Rather, predestination needs to be Christ shaped all the way down: the elect united with Christ, figuring His death and resurrection, and the reprobate figuring the shadows Christ's passion cast (i.e. the forms of Pilate, Herod, Judas, the Temple Hierarchy, the mocking and sneering mob of Pharisees and peasants). Augustine is still too much enthralled to Plotinian metaphysics. But, when it comes down to the debate with Pelagius, he is absolutely right. While we were dead in our sins, lost and rebellious, God intervened and saved us. Soli Deo Gloria.
Sacraments: Generally, I find Augustine a helpful expositor of "sacraments", maintaining both the materiality of God's provision, while also not lapsing into a kind of idolatry. This depends upon his view of semiotics and the distinction, but unity, of sign and thing signified. Per a Biblical example, the bronze serpent Moses held in the desert actually saved. If Israelites did not look to it, they would've died from the snake venom. However, when a temptation to venerate the bronze serpent takes hold of Jerusalem, Hezekiah had the bronze serpent melted down (c.f. 2 Kings 18:4). God is present in material artifacts, as He commands and promises, but they are not reducible to Him. However, Augustine's mechanical notion of grace is bizarre, and has become a justification for a warped view of ordination and apostolic succession.
Trinity & Triads: At this point, Augustine is mostly a roadblock. In short, Augustine was able to find a way to integrate Neo-Platonic triadology and Nicaean Christology through a working definition of person as relation. While Augustine remains staunchly anti-Arian, he has more in common than he'd like to admit. This would not be such a problem if Augustine was not so influential in what became Latin theology. As can be seen in both texts and arts, Augustine's theology resulted in a reductive role for the Holy Spirit as vinculum caritatis, the binding cord of love between the Father and the Son. While the filioque is not itself a problematic phrasing, its interpretation followed a Neo-Platonic scheme that has been unable to understand unity and distinction without the creation of a fourth thing. There is no genus of God that lays behind the back of the Persons that we can more simply refer to. The New Testament did not merely make an addition to the more simple formula of one God in the Old Testament. The Trinity is not merely an irrational numbers game, 1+1+1=1. The Apostles clearly have no problem of intelligently describing things without throwing their hands up in confusion. Perhaps our Hellenistic metaphysics are defunct and incapable, which is exactly what Athanasius argued. I think Augustine's Neo-Platonic articulation of Nicaea only caused more problems later down the line. We'd be better off turning to Athanasius and Gregory Nazianzen for better thoughts.
So, all in all, Augustine has left much of a legacy, both good and bad, in his wake. However, he was also a great pastoral figure, and is worthy of emulation in how he combined intelligence, devotion to the Scriptures, and ordained leadership. To conclude, I will quote from Peter Brown's biography of the African bishop. May it stir your soul as it did mine, and continues to do.
"For Augustine’s doctrine of predestination…was a doctrine for fighting men. A monk might waste his leisure worrying about his ultimate identity: to Augustine, such an anxiety was misplaced. A doctrine of predestination divorced from action was inconceivable to him. He had never written to deny freedom, merely to make it more effective in the harsh environment of a fallen world."
"Predestination…had only one meaning for Augustine: it was a doctrine of survival, a fierce insistence that God alone could provide men with an irreducible inner core."