Constantine the Great
Disorder ruled the day in the 3rd century Roman Empire. As a young military commander, Constantine was a pagan but, while marching south through Italy, he claimed to have had a vision of the Christian "chi-rho" sign across the face of the sun, with the message, In this sign you will conquer. With this encouragement, he went on to capture Rome and control the empire.
In gratitude, he declared his allegiance to the Christian God, although he continued to permit pagan worship within the areas under his control.
Although it can be effectively argued that Constantine's faith was a matter of political convenience rather than a spark of faith, it cannot be argued that Christianity became well established into the culture of society during his rule, and throughout most of the remainder of the history of the Roman empire.
From Persecution to Privilege
Except for one, every emperor to follow Constantine tolerated Christianity, and many embraced it. With this change in status, Christianity emerged from centuries of oppression and persecution to become a favored religion.
The majority of those within the Roman Empire continued to hold allegiance to paganism, however.
Constantine gave the Roman Empire a new capital - Constantinople, previously known as Byzantium, now called Istanbul, and strategically located near the point where Asia and Europe meet. In Constantinople, Constantine built a great cathedral, named Santa Sophia, which was destroyed by fire in the 6th century.
Although not the state religion, Christianity had become an important part of the fabric of the empire. Christians were quicker to advance in service of the empire, and other privileges were offered to believers. A later emperor, Theodosius, penalized non-Christians with laws similar to those that were once enforced against Christians.