Final answer:
Pope St. Gregory the Great took the title 'Servus servorum Dei,' meaning 'Servant of the Servants of God.' He did not rely on military power but on diplomacy and Christianity expansion to assert papal authority.
Step-by-step explanation:
Pope St. Gregory the Great took on the title Servus servorum Dei, which translates to 'Servant of the Servants of God'. His authority as pope was derived not from military might but through the development of relationships with kings and the spread of Christian missionary work. Popes like Gregory advanced the Church's influence using their spiritual authority to create alliances and spreading Christianity.
Gregory's era did not yet see the pope as the undisputed spiritual head of the entire Church, and he navigated a complex political landscape to increase the independence and authority of the papacy within Christendom. This entailed engaging with secular rulers and deploying missionaries to convert and bring new areas under the church's influence. The resulting relationships often provided subsequent popes with precedents to cite when asserting their authority, such as the coronation of Charlemagne by Pope Leo III, and the era's ecclesiastical documents like the later-forged 'Donation of Constantine.'