Wishing to worship the Supreme Being, Emperor Constantine the Great of Rome (272 - 337 AD) converts to Christianity and simultaneously changes world history.

Colossal marble head of Emperor Constantine the Great, Roman, 4th century

Constantine's Testimony

Constantine the Great, mosaic in Hagia Sophia, c. 1000

"To acknowledge ... in solemn terms the beneficence of the Supreme Being is by no means boasting. He searched for and chose my service to carry out his purpose. Starting ... at the faraway Britannic sea and the regions where the sun ... sets, by the help of the Supreme Power, I drove out and scattered all the prevailing evil things, in order that the human race, reared with my assistance, might call upon the service of the holy law ... I am firmly convinced that I owe my life and every breath ... to the Supreme God."

Constantine's Letter to King Sapor of Persia

Colossal head of Constantine, from a seated statue: a youthful, classicising, other-worldly official image (Metropolitan Museum of Art)^[

I profess the most holy religion.  I confess that as a disciple of the Holy God I observe this worship.  With the power of this God on my side to help me, beginning at the boundaries of the Ocean, I had gathered every nation, one after another, throughout the world, to the certain hope of salvation ... This God I worship and my army is dedicated to him and wears his sign on their shoulders, marching directly wherever the cause of justice summons them.  I confess that I honour this God with never-dying remembrance, this God in the height of his glory I delight to contemplate with a pure and simple heart.

The Conversion of Constantine

Constantius appoints Constantine as his successor by Peter Paul Rubens, 1622

Throughout the fourth century, relations between the church, the emperor, and pagan religion were changing continually.  Constantine's defeat of Maxentius at the Battle of the Milvian Bridge in the autumn of 312, and his interpretation of that victory as the response of the Christian God to his prayer for help, propelled church and state into a new age for which neither was prepared.  Out of this new relationship between Christian church and Christian emperor stemmed the turbulent history of church/state relations in the later Roman Empire and throughout the Middle Ages.

Constantine prays to God.  The response from God is a cross in the noonday sky

The Battle of the Milvian Bridge by Giulio Romano

Constantine's account of his conversion, told by the emperor himself to the Church historian Eusebius of Caesarea towards the end of his life, is well known.  Constantine, alarmed by reports of Mazentius' mastery of magical arts, prayed to the "Supreme God" for help.  The response was a sign, a cross in the noonday sky 'above the sun', and with it the words, 'Conquer by this.'  That night Christ appeared to Constantine in a dream and commanded him to use the sign -- apparently Chi-Rho, the initial letters of the name of Christ -- 'as a safeguard in all engagements with his enemies'.  According to the historian Lactantius, Constantine put this sign on the shields of his soldiers, and then marched on Rome, confronted Maxentius -- who was miraculously induced to fight outside the city fortifications -- and conquered.

Though converting to Christianity, Constantine maintains pagan customs

The Baptism of Constantine, as imagined by students of Raphael

This story has been doubted.  But Constantine's attitude towards the Christian church after he became emperor, and his new laws, demonstrate that his allegiance to Christianity was genuine, though his understanding of the Christian faith was at first no doubt imperfect.  Indeed, Constantine did retain the pagan high priest's title of Pontifex Maximus; for a decade his coins continued to feature some of the pagan gods, notably his own favourite deity, the Unconquered Sun; and he delayed Christian baptism until the end of his life.  However, delayed baptism was the custom of the age, a device for avoiding mortal sin; and retaining the pagan symbols was a necessary compromise with his pagan subjects, still very much in the majority.

A gold multiple of "Unconquered Constantine" with Sol Invictus, struck in 313 AD. The use of Sol's image appealed to both the educated citizens of Gaul, who would recognize in it Apollo's patronage of Augustus and the arts; and to Christians, who found solar monotheism less objectionable than the traditional pagan pantheon.

Constantine treated Christianity as the favoured, though not yet the official, religion of the Empire.  He granted immunities to the clergy and lavished gifts on the church; in his letters and edicts he spoke as if the Christian God were his own.

Constantine's previous religion was the worship of the Unconquered Sun

A representation of Jesus as the sun-god Helios/Sol Invictus riding in his chariot. Mosaic of the 3rd century on the Vatican grottoes under St. Peter's Basilica.

It is important to understand Constantine's previous religion, the worship of the Unconquered Sun.  If the story of the cross in the sky is true, he may have interpreted the sign as his own special deity commending the worship of the Christian God.  Perhaps Constantine continued to identify the sun with the Christian God in some way -- a belief made easier by the tendence of Christian writers and artists to use sun imagery in portraying Christ.  For them, Christ is the source of light and salvation; a mosaic from a third-century tomb found under St. Peter's, Rome, even shows him as the sun god in his chariot.  When in 321 Constantine made the first day of the week a holiday, he called it 'the venerable day of the Sun' (Sunday).

Constantine and his mother build churches

Church of the Holy Sepulchre in Jerusalem

Another result of Constantine's conversion was renewed interest in the Holy Land by people in the West.  Since the failure of the Second Jewish Revolt of Bar Kokhba (132-135), Jerusalem had been a pagan city; Constantine and his mother Helena now made it Hadrian's Temple of Venus, and Helena discovered what was believed to be the 'True Cross' on which Jesus had been crucified.  Here -- with the Church of the Holy Sepulchre -- and elsewhere, Constantine and Helena built churches, and pilgrims came in increasing numbers to the holy places.

Christianity and Pagan Customs

Ave, Caesar! Io, Saturnalia! (1880) by Lawrence Alma-Tadema

The Christian church began to take over pagan ideas and images.  From sun-worship, for example, came the celebration of Christ's birth on December 25, the birthday of the sun.  Saturnalia, the Roman winter festival of December 17-21, provided the merriment, gift-giving, and candles typical of later Christmas holidays.  Sun-worship hung on in Roman Christianity, and Pope Leo I, in the mid-fifth century, rebuked worshippers who turned round to bow to the sun before entering St. Peter's Basilica in Rome.  Some pagan customs which were later Christianized, for example the use of candles, incense, and garlands, were initially avoided by the church because they symbolized paganism.

Sources:

Dowley, Tim, ed. Introduction to the History of Christianity, Second Edition. Minneapolis: Fortress Press, 2013, pages 104-106.

Eusebius of Caesarea, Life of Constantine 2.28-29 and 4.9, trans. Paul Keresztes, in "Constantine: Called by Divine Providence" in Elizabeth A. Livingstone, ed. Tudia Patristica (Kalamazoo, MI: Cistercian Publications, 1985).