1. Wicked monarchs, like Henry VIII, tried (unsuccessfully) to destroy the Church, to help them achieve their own twisted ambitions. Even today some politicians are anxious to turn Britain into a pagan humanist country.
2. Byzantine Empire had arisen from the ruins of the (Eastern) Roman Empire as the most consolidated, organized and stable province from all of the Romans' civilizations. But in its a greatest times, it was barely shallow of the Roman Empire, having about 1/2 of the population and 1/3 of the territory. The fortune of the Eastern Roman Empire was rather luckier than the western part and it never lost its ruling elite, senate, aristocracy and dynasty as it had happened to Rome. The sense of direct lineage of various Eastern Roman institutions was helping to survive and transform into new medieval state. Rome lost not only the dynasty, but also senate and most of the aristocracy. Without the ruling elite, there was also very limited drive to maintain army and once strong and feared roman legions were history. On top of it, during the last decades of the Roman empire, its navy disintegrated and was unable to secure Mediterranean that was a vital for the Roman economy. On the other hand, Greek-Byzantine navy was consolidated and remained the most powerful in the world until Crusades.
Beside army and navy that was able to protect Constantinople, Byzantine territory was not entirely overrun by migrating nations and its leadership never allowed foreigners to take control of command as was in the case of the west. Byzantine state was much smaller and never overextend its defenses. The lost wars of Justinian and inconclusive and nothing bearing conquests of his reign taught entire generations of rulers not to expand the Byzantine territory too far. The Balkans was taken by Slavs, Egypt and Levant by Persian and Arabs, so Byzantine was left with smaller, but defensive territory. The downfall of Byzantine Empire started, when all its resources were invested into conquering Bulgaria in 1018 and it has never reaped the benefits of extending the empire. Byzantine empire was one of the first states in history that used diplomacy to prevent conflicts and negotiate treaties as one of the means of survival of the state.
The one of the last element that helped Byzantine empire to outlive Rome was the location of the capital itself that was considered unconquerable until IV Crusade. Constantinople was a terminus of Asian long distance trade and a financial capital of the Mediterranean and Europe where various trading goods were exchanged and thus generating a significant profit for the treasury. The fall of Constantinople coincide with the rise of Venice and Genoa and was closely related with economical decline of the city. When Constantinople was undisputed commercial center of the world, it has unique position among economically depressed cities of the post-roman era. Once this position was challenged, the fortune of the state was in peril and eventually lead to end of the Byzantine state.