What were the contributions made to political philosophy by St. Augustine?

Answer 1

I think you need to read about him first and see what he dealt with:

http://www.newadvent.org/cathen/02084a.htm

http://www.geocities.com/Athens/1534/august/augartic.htm

St. Augustine stands as a powerful advocate for orthodoxy and of the episcopacy as the sole means for the dispensing of saving grace. In the light of later scholarship, Augustine can be seen to serve as a bridge between the ancient and medieval worlds. A review of his life and work, however, shows him as an active mind engaging the practical concerns of the churches he served.
More here:http://www.ccel.org/a/augustine/

Augustine remains a central figure, both within Christianity and in the history of Western thought, and is considered by modern historian Thomas Cahill to be the first medieval man and the last classical man.[2] In both his philosophical and theological reasoning, he was greatly influenced by Stoicism, Platonism and Neoplatonism, particularly by the work of Plotinus, author of the Enneads, probably through the mediation of Porphyry and Victorinus (as Pierre Hadot has argued). His generally favorable outlook upon Neoplatonic thought contributed to the "baptism" of Greek thought and its entrance into the Christian and subsequently the European intellectual tradition. His early and influential writing on the human will, a central topic in ethics, would become a focus for later philosophers such as Schopenhauer and Nietzsche. In addition, Augustine was influenced by the work of both Virgil (known for his teaching on language) and Cicero (known for his teaching on argument
(from Vikpedia)

Answer 2

Start by saying he is not a political thinker. But in The city of God he divides people into to groups of citizens, the ones from the city of men and those who belong to the city of God. with that contribution he supports to main powers, God's and men.