Final answer:
The spread of both Roman Catholicism and Orthodox Christianity was significantly aided by monasticism, which provided a peaceful yet enduring means by which Christianity could flourish throughout Europe and beyond.
Step-by-step explanation:
The event that helped the spread of Roman Catholicism and Orthodox Christianity was d. monasticism. Monasticism played a crucial role in the mission of Christianizing Europe, which included outreach to pagan European tribes, education, and preservation of classical texts. Monasteries served as centers of learning and were instrumental in the process of conversion because they were isolated from the general population, enabling monks to develop and spread their beliefs without direct opposition. This aided in the establishment of Christian institutions and practices throughout Europe and beyond.
It's important to note that while events like the Crusades and Barbarian invasions certainly had significant effects on the historical trajectory of Christianity, they were more direct in their impact and often associated with violence and conflict. In contrast, monasticism provided a peaceful and enduring means by which Christianity could flourish.