Stoicism is a school of Hellenistic philosophy, founed in Athens by Zeno of Citium in the early third century BCE. It proved to be a popular and durable philosophy, with a following throughout Greece and the Roman Empire from its founding until all the schools of philosophy were ordered closed in 529 CE by the Emperor Justinian I, who perceived their pagan character to be at odds with his Christian faith. The core doctrine of Stoicism concerns cosmic determinism and human freedom, and the belief that virtue is to maintain a will that is in accord with nature.