Simplify the following text for a Grade 14 native English-speaking person.
Original: Stoic doctrine was a popular and durable philosophy , with a following throughout Greece and the Roman Empire , from its founding until the closing of all philosophy schools in 529 AD by order of the Emperor Justinian I , who perceived their pagan character to be at odds with the Christian faith .
Simplified: Stoic doctrine was popular in Greece and the Roman Empire from its founding until all the schools of philosophy were ordered closed in 529 AD by the Emperor Justinian I , who perceived their pagan character to be at odds with his Christian faith .