The medieval period has generally been reckoned to have lasted from the Fall of Rome to the Reniassance. For the Fall of Rome, any dates between 312 (the Battle of Milvian Bridge) and 476 CE (deposition of the last Roman Emperor, Romulus Augustulus) are reasonable. The Renaissance began in Italy in the 14th century, and in northern Europe at the end of the 15th.
Augustine could therefore be seen as a medieval figure. I grant that scholars nowadays speak of Late Antiquity as its own distinct and transitional period, but I would have to argue that by the time of his death in 430 CE, the Middle Ages had begun in many areas of Europe.
Your point that he was not really a European, however, is well taken.
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