Final answer:
Emperor Constantine's conversion to Christianity and his implementation of the Edict of Milan in 313 were his most crucial accomplishments, leading to the legalization and expansion of Christianity, which had a profound influence on the course of Western civilization.
Step-by-step explanation:
Emperor Constantine's most important accomplishment was his conversion to Christianity and the Edict of Milan, which granted religious tolerance. This monumental event transformed the role and status of Christianity within the Roman Empire. By issuing the Edict of Milan in 313, Christianity was legalized, and its adherents were allowed to begin building churches.
This shift marked the end of state-sanctioned persecution of Christians, allowing for a significant increase in the number of Christian followers and the power of Christian institutions.
Constantine's sponsorship and formal conversion to Christianity, which is widely acknowledged among historians, led to the expansion of Christianity throughout the empire.
This strategic alliance between the imperial power and the Christian Church fundamentally changed the historical trajectory of Europe and the cultural landscape of the Western world. Furthermore, the establishment of Constantinople as a new capital city marked both a political and cultural shift, setting the stage for the rise of Byzantine influence.