That sir is an astute question .
As my Orthodox friend has said, we don't follow the teachings of one particular saint. THAT is picking and choosing FOR YOURSELF what to believe. Christianity is a revealed religion. It is GIVEN to us. We do NOT "decide" what God said to us, we are taught it through men guided by the Spirit. Thus, the Church looks to ALL the Tradition, not just one man, even a saint. St. Augustine's teachings on predestination were not ALL accepted, I believe 10 of 18 distinct teachings. However, the Church Councils, the final arbiter of what we believe, disagreed with ALL of St. Augustine's teachings, esp. mass damnatia - the idea that ALL unbaptized infants are punished in hell. Even his disciple Prosper of Aquitaine could not handle some of St. Augustine's teachings - even though he DID defend SOME of them.
Clearly, Saint Augustine said a lot on the subject. And some of it disagrees with your ideas on predestination - such as men are secondary causes of their actions and men are responsible for turning to God to be saved.
The Spirit speaks to the Church through the consensus of Tradition. He speaks through the sense of the faithful, through the Liturgy, through the daily activities of the people of God. When men hear such teachings that go too far in contrasting Pelagius, our "sense" rebels against it. We bind ourselves to the Council's decisions on such matters, not one Saint.
Now why do you select St. Augustine over the rest of Christendom on the subject that God predestines the reprobate actively to hell? Do you ALREADY have that belief and are looking for "proof" from ancient Christendom to "verify" your already-held point of view?
Regards