Final answer:
Pope Gregory called Clovis the Constantine of the West because both leaders used Christianity to fortify their rule and to bind their territories under a unified religion, thereby legitimizing and strengthening Christian authority in their realms.
Step-by-step explanation:
Pope Gregory labeled Clovis as the Constantine of the West because both rulers played pivotal roles in establishing and legitimizing Christian authority within their respective empires. Clovis, like Constantine, used Christianity as a political tool to consolidate his power and to gain the favor of his Roman subjects by converting to Latin Christianity. This shrewd move enabled him to unite the Franks and the Gallo-Roman population under one faith, thus providing a common religious ground to strengthen his rule. Similarly, Constantine was the first Roman emperor to convert to Christianity, which not only facilitated the spread of Christianity but also tied the church to the state. Gregory and other popes emphasized these parallels to bolster the papacy's claims to political as well as spiritual authority in the West.