Which emperor is known as 'Constantine the Great'? (r. 306-337 AD)

The story behind the answer

Constantine I is called “Constantine the Great” (r. 306–337). He legalized Christianity with the Edict of Milan (313), convened the Council of Nicaea (325), moved the imperial capital to Byzantium, refounding it as Constantinople, and defeated his rival Maxentius at the Battle of the Milvian Bridge (312) after the famous Chi-Rho vision. These actions reshaped Rome and the future Byzantine Empire.

To avoid confusion: Constantine II was his son who briefly ruled after 337; Constantine III was a short-lived usurper in the West; Maxentius was the opponent Constantine I overthrew. Memory tip: “I = Initiates”—Constantine I initiated the Christian empire and a new capital, making him the one remembered as “the Great.”