Final answer:
The First Crusade bolstered papal leadership by showcasing the pope's ability to mobilize Christians for a holy war, extending papal influence over the nobility, and enhancing the pope's reputation as a spiritual leader. This contributed to the perception of the papacy as a strong political and religious authority in Western Christendom.
Step-by-step explanation:
The First Crusade significantly contributed to the growth of papal leadership in a few key ways. Firstly, Pope Urban II's call to holy war underscored the papacy's capacity to mobilize Christians across Europe for a common religious cause. By leading such a grand venture, the papacy reinforced its position as the spiritual and political leader of Western Christendom.
Moreover, the crusading movement provided the papacy with an opportunity to assert its influence over the aristocracy and monarchs.
Despite often having unreliable support from noble protectors, the pope was able to rally thousands to the religious cause, thus demonstrating the papal ability to direct Christian efforts and spiritual endeavors beyond the control of secular rulers.
Finally, while the First Crusade itself saw mixed outcomes—including egregious actions in contrast to its spiritual ideals—it forged an enduring vision of the pope as a reformer and champion of the church's interests against secular interference. It was an assertive demonstration of the papacy's ability to direct Christian action, and in the short term at least, it enhanced the reputation and moral authority of the pope.