RIC VI, Ticinum, no. 124a, p. 298.

This nummus, minted between 312 and 313 CE at Ticinum, Italy, seat of the imperial mint from 274 until 327 CE, depicts on the obverse the head of Constantine and on the reverse Mars, the Roman god of war (about the denomination itself see). The inscription on the obverse refers to Constantine as Constantinus, Pius, Felix, Augustus, the latter serving to emphasize the emperor’s sacral standing.
By 312 CE, when this issue was minted, Constantine ruled over Spain, Gallia, Britannia, Africa, and Italy, in fact, the whole Latin West. Between the years 310 and 312 CE, Constantine had waged a victorious war against the son of Maximianus, Maxentius, a usurper who had ruled territory which included central and northern Italy, as well as the north African provinces. According to Lactantius (On the Death of the Persecutors XLIV.4-6), during the night which preceded the final clash against Maxentius’s army, Constantine dreamed that in order to defeat his enemy, he had to trace on the shields of his soldiers the letter X, which referred to Christ. On the other hand, Eusebius narrates that Constantine had a vision in the middle of the day. He perceived “a trophy of the cross…, carrying the message, ‘by this sign, conquer/prevail’” (Eusebius of Caesarea, Life of Constantine I.28). The final clash came the day after at the Milvian Bridge, near Rome. The army of Maxentius was utterly defeated, and Constantine could enter Rome.
According to Augusto Fraschetti, when Constantine entered the city in 312 CE, he celebrated a triumph that was devoid of pagan pageantry (Fraschetti, La conversione da Roma pagana a Roma cristiana). Yet, this issue, which depicts Mars on the reverse, clearly was minted after Constantine’s “conversion.” Mars was the god of war. In Roman religion, he was second in importance only to Jupiter and Neptune. As the husband of Rhea Silvia and the father of Romulus and Remus, he played a prominent role in the foundation of Rome. Besides, he was the most prominent of the military gods in the religion of the Roman army. Most of his festivals were held in March, the month named for him, Martius, and in October, which began the season for military campaigning and ended the season for farming. Under the influence of Greek culture, Mars was identified with the Greek god Ares. On this coin, the god is depicted holding a shield and a reversed spear. This iconography differs from the traditional depiction of the god, advancing naked, holding a trophy and a spear. In fact, on this issue Mars bears a strong Roman character, while on other issues he follows the iconography of Ares, his Greek counterpart. Mars held a special meaning for Augustus, the first emperor, as he was considered to be an ancestor of the Julian family, of which Octavian became a member through adoption. In 147-148 CE, Antoninus Pius celebrated the nine-hundredth anniversary of the foundation of Rome through the minting of different issues, which depicted scenes associated with the birth and the beginning of Rome. Mars advancing towards Rhea Sylvia is one of the chosen figures. Antoninus Pius wished to emphasize his association with Rome’s faraway past. Later on, during the Severan period and in the third century, Mars was once more perceived as a warrior god, who brought victory to the Roman army.
The depiction of Mars on the reverse of this issue does not have any apparent link to Rome’s past, but it emphasizes the martial achievements of Constantine. Thus, the inscription Mars Conservator, or “Mars the keeper,” emphasizes the protective character of the god. Until 335 CE, Constantine continued to mint coins depicting the god Mars on the reverse. It may seem problematic to explain the survival of the depiction of Mars, the god of war, on the reverse of coins minted by an apparently Christian ruler. However, there is much evidence to suggest that even after his conversion, Constantine maintained links, particularly through his portrayal on numismatics and in portraiture, to pagan religion. Indeed, the way in which he was represented was arguably often ambiguous; he is famously depicted gazing heavenward, which not only echoed depictions of Alexander the Great, but also hinted at his connection to the Christian God for those who would interpret it this way (see the discussion of this issue in the commentaries for the Colossus of Constantine and the Life of Constantine IV.15). According to Clive Foss, from 316 CE onwards, a cross in the field was added to the reverse of coins like the present issue, possibly emphasizing the emperor’s religious choice (Foss, Roman Historical Coins, p. 285). In any case, there is no doubt that this issue celebrates a pagan god. Hence Constantine’s coinage cannot be seen as necessarily reflecting a complete break from his former religion upon his uptake of Christianity.