What accounted for the changes in the Church from the end of the Roman Empire through the High Middle Ages?
During this time, the power of the Church generally increased across Europe though it was starting to decline as the Middle Ages came to an end. The main reason for this was the fact that society had become politically fragmented. In Europe for most of the Middle Ages, political power was split up not just between countries but between various lords in the feudal system. This meant that there was very little centralization of power.
Into this vacuum stepped the Church. It was the one centralized entity in Europe that could give Europeans a sense of unity. It was also seen as the only way to God and to a good afterlife. For these reasons, the Church was able to wield a great deal of power. It was a large, centralized organization that was able to use its spiritual power to gain temporal wealth and power. This power generally expanded from the fall of Rome, but it was starting to fade by the end of the Middle Ages as things like the Great Schism reduced the moral authority of the Church.