Final answer:
The Catholic Church maintained unity in Western Europe after the Roman Empire's demise by providing religious structure, conserving Roman legacies, and supporting new rulers, thereby fostering a common Christian culture.
Step-by-step explanation:
After the fall of the Roman Empire, the Catholic Church became the principal unifying force in Western Europe, providing a consistent structure amidst political fragmentation. As the primary institution to preserve the legacy of ancient Rome, the church united the region under shared religious beliefs and Christian identity. The Church's administrative hierarchy, with the pope at its apex, provided organization and leadership in a time where political entities were largely fragmented and decentralized. Through the preservation of classical traditions, conversion efforts aimed at Germanic tribes, and the legitimization of rulers, the Church played a pivotal role in transforming Europe from a collection of warlord territories into a civilization with a shared religious and cultural foundation, thus laying the groundwork for medieval culture.