Final answer:
The concept of Christendom emerged in the Christian mind as a result of the Germanic invasions in Europe after the fall of the Roman Empire.
Step-by-step explanation:
The idea that emerged in the Christian mind as a result of the Germanic invasions was the concept of Christendom. Christendom refers to the idea of a single, unified empire of Christianity that became central to the identity of Christians in Europe after the fall of the Roman Empire. The Germanic Kingdoms, along with the western empire and other rising empires, adopted this idea as they embraced Christianity and saw Islam as an enemy due to the Arab conquests.