Constantine I (272 –  337 CE) was a powerful general who reigned over the Roman Empire as emperor until his death. He made the previously named city Byzantium (now Istanbul, Turkey) capital of the whole Roman Empire. As emperor, he named the city Constantinople, which means "City of Constantine" in Greek.

Before Constantine became Emperor, he was fighting for the throne at the Battle of Milvian Bridge over the Tiber River. When he saw a cross in the sky with the words in hoc signo vinces (Latin for "in this sign you shall conquer"), he changed his deity from Apollo to Jesus and won the battle.

In pagan Rome before this, it had been against the law to believe in Christianity, and Christians had often been tortured or killed. Constantine protected them. He went on to organize the whole Catholic Church at the First Council of Nicea, even though he himself did not get baptized until near the end of his life. He wrote the Edict of Milan that made Christianity legal in the empire.

edited from: https://simple.wikipedia.org/wiki/Constantine_the_Great