Although polytheism had been suppressed by the state since at least the time of Constantine in the 4th century, traditional Greco-Roman culture was still influential in the Eastern empire in the 6th century. Hellenistic philosophy began to be gradually amalgamated into newer Christian philosophy. Philosophers such as John Philoponus drew on Neoplatonic ideas in addition to Christian thought and empiricism. Because of the active paganism of its professors, Justinian closed down the Neoplatonic Academy in 529. Other schools continued in Constantinople, Antioch, and Alexandria, which were the centers of Justinian's empire.