St. Constantine, also known as Constantine the Great, was a Roman emperor who ruled from 306 to 337 AD. He is best known for being the first Roman emperor to convert to Christianity and for issuing the Edict of Milan in 313 AD, which granted religious tolerance throughout the empire. His reign marked a significant turning point in the history of Christianity. Constantine also founded the city of Constantinople, which later became the capital of the Byzantine Empire. He played a crucial role in the development of the early Christian Church and convened the First Council of Nicaea in 325 AD, which addressed key theological disputes and helped unify Christian doctrine.