BY THE BEGINNING OF THE FOURTH CENTURY CE, the religious life of the Romans was in flux. Inevitably, this was reflected in coinage, where designs often depicted deities. Although the 12 Olympian gods[1] were still venerated in state-funded temples, the common people increasingly practiced a variety of "Eastern" or "mystery" religions. These included worship of the Egyptian goddess Isis; Mithras, a divinity of Persian origin who was especially popular with soldiers; and Serapis, a syncretic "Graeco-Egyptian" god invented by the Ptolemaic dynasty in the third century BCE.

And at a certain risk to themselves, a small minority turned to Christianity.

For even though pagan polytheism generally accepted imported gods, the growing population of Christians was persecuted or tolerated depending on the whim of the current ruler[2]. Christians (along with the Jewish minority) insisted that there was only one God, all others being either demons or false idols. Participation in official rituals showed allegiance to the state; refusal was a kind of treason.

Constantine the Great
In 312, Constantine invaded Italy to fight the rival army of Maxentius. Just before the battle of the Milvian Bridge outside Rome (28 October 312), Constantine had a vision:

In gratitude for victory, Constantine adopted the Christian God as his patron, but as a politician and commander, he continued to observe the traditions of his predominantly pagan subjects and soldiers. The Edict of Milan in 313 legalized Christianity throughout the empire, though persecution of Christians had largely ended with the death of Galerius in 311. The first appearance of the Christogram on coinage, datable to 315, is a rare silver miliarensis[6]. The tiny symbol appears on the crest of Constantine's elaborate jeweled helmet.

Magnentius

Theodosius the Great
The dynasty of Theodosius (379 - 457) completed the eradication of paganism in the empire.

Theodosius I ("The Great") decreed that Christianity would be the only legal religion in the Empire (Jews were grudgingly tolerated, with sporadic episodes of official persecution). Remaining pagan temples were closed, and the imperial treasury confiscated their wealth. The eternal fire tended by the Vestal Virgins in Rome was extinguished. The Olympic Games were banned in 393 because athletes competing naked offended Christian sensibilities.

The Cross, which horrified pagan Romans as an instrument of execution for criminals, gradually began to appear as a major design element on coins. For example, the reverse of a solidus of Honorius, son of Theodosius, shows an angel (re-imagined from the pagan Victoria) beside a tall jeweled cross. Coins struck in the names of several Theodosian empresses show the cross, surrounded by a wreath on the reverse.

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CoinWeek Ancient Coin Series: The Christianization of Roman Coinage(www.coinweek.com/featured-news/coinweek-ancient-coin-series-christianization-roman-coinage/)