Following the conquest of Asia Minor by Alexander the Great, the rest of Asia Minor was opened up to Greek settlement. Upon the death of Alexander, Asia Minor was ruled by a number of Hellenistic kingdoms such as the Attalids of Pergamum. A period of peaceful Hellenization followed, such that the local Anatolian languages had been supplanted by Greek by the 1st century BC. Asia Minor was one of the first places where Christianity spread, so that by the 4th century AD it was overwhelmingly Christian and Greek-speaking. For the next 600 years, Asia Minor and Constantinople which eventually became the capital of the Byzantine Empire would be the centers of the Hellenic world, while mainland Greece experienced repeated barbarian invasions and went into decline.