The book starts with Constantine taking over as ruler of the Roman Empire. At this point, the previous emperor, Maxentius has died, but the two "under rulers", Licinius and Maximinus Daia are still in power. Constantine offers his half sister in marriage to Licinius to get rid of Maximinus. It works. The two get married, and Constantine declares Christianity legal in the empire. Later, Constantine has Licinius and his wife banished to Thessaloniki and later killed.
Big things that happen--
Christianity is made legal in the Roman Empire
The Council of Nicaea meets and pens the Nicene Creed, which is still the definition of Christianity today
Constantine moves the capital of the Empire to Byzantium, including the governing people and many of the art works and relics from all of the great cities