The relative political and religious unity Justinian’s campaigns brought back to Byzantium declined steadily after his death. For almost 1,000 years, the two kinds of Christianity - later called "Catholic" and "Eastern Orthodox," although both terms speak to the idea of one universal and correct form of Christian doctrine - were sundered by the great political divisions between the Germanic kingdoms of the west and Byzantium itself in the east. In Eastern Europe, small kingdoms and poor farmers played host to rival missionaries preaching the slightly-different versions of Christianity. Trade existed, but was never as strong as it had been during the days of the united empire.
Byzantium’s major ongoing problem was that it faced a seemingly endless series of external threats. Byzantium was surrounded by hostile states and groups for most of its existence, and it slowly but steadily lost territory until it was little more than the city of Constantinople and its immediate territories. It is important to remember, however, that this process took many centuries, longer even than the Roman Empire itself had lasted in the west. During that time, Constantinople was one of the largest and most remarkable cities on the planet, with half a million people and trade goods and visitors from as far away as Scandinavia, Africa, and England. Its people believed that their empire and their emperor were preserved by God Himself as the rightful seat of the Christian religion. Thanks to the resilience of its people, the prosperity of its trade networks, and the leadership of its emperors (the effective ones, anyway), Byzantium remained a major state and culture for centuries despite its long-term decline in power from the days of Justinian.
The most significant leader after Justinian was the emperor Heraclius (r. 610 – 641). He was originally a governor who returned from his post in Africa to seize the throne from a rival named Phocas in the midst of a Persian invasion. The empire was in such disarray at the time that the Persians seized Syria, Lebanon, and Egypt, cutting off a huge part of the food supply to Constantinople. In the process, the Persians even seized the “True Cross,” the cross on which (so Christians at the time believed) Christ Himself had been crucified, from its resting place in Jerusalem. Simultaneously, the Avars and Bulgars, barbarian peoples related to the Huns, were pressing Byzantine territory from the north, and piracy was rife in the Mediterranean.
Heraclius managed to save the core of the empire, Anatolia and Greece, by recruiting free peasants to fight instead of relying on mercenaries. He also focused on Anatolia as the breadbasket of the empire, temporarily abandoning Egypt but keeping his people fed. He led Byzantine armies to seize back Jerusalem and the True Cross from the Persians, soundly defeating them in 628, and in 630 he personally returned the True Cross to its shrine in Jerusalem. The fighting during this period was often desperate - Constantinople itself was besieged by an allied force of Avars and Persians at one point - but in the end Heraclius managed to pull the empire back from the brink.
Despite his success in staving off disaster, however, a new threat to Byzantium was growing in the south. The very same year that Heraclius returned the True Cross to Jerusalem, the Islamic Prophet Muhammad returned to his native city of Mecca in the Arabian Peninsula with the first army of Muslims. Heraclius had no way of knowing it, but Byzantium would soon face a threat even greater than that of the Persians: the Arab caliphates (considered in the following chapter). Indeed, Heraclius himself was forced to lead Byzantium during the first wave of the Arab invasions, and despite his own leadership ability vital territories like Syria, Palestine, and Egypt were lost during his own lifetime (he died in 641, the same year that most of Egypt was conquered by the Arabs).