Yet, one must also consider ulterior motives for Constantine's conversion. He chose to embrace Christianity shortly before a large battle against another Roman ruler for total control over the Western Roman Empire. With the stakes so high it is conceivable that Constantine converted in hopes that the Christian God would aid him in battle. This explanation is supported by the contemporary account of the Battle of Milvian Bridge written by a Christian named Eusebius. Eusebius recounts that Constantine had a vision of a cross above the sun with the words "Conquer by this" engraved across it. Eusebius claims that Constantine instructed his men to represent this symbol on their shields and banners by superimposing the letters "Chi" and "Rho" on top of each other:
Thus, Eusebius clearly saw Constantine's conversion as a means of achieving military victory.
Alternatively, Constantine may have hoped to gain political capital by converting to Christianity. Two years prior to Constantine's conversion Galerius, another ruler in the triumvirate of the Western Roman Empire, ended the Great Persecution of Christians in the Empire by making it legal to practice Christianity. Constantine may have sensed the fragility of the balance between Christianity and paganism. He also might have realized that he stood to gain the support of an increasing number of Christians within the Empire by declaring it the official religion of the Empire.
Which of these three explanations is most plausible? Perhaps a second account of the Battle of Milvian Bridge, authored by Lactantius, offers the most telling information. Five years after the battle he wrote:
Constantine was directed in a dream to cause the heavenly sign to be delineated on the shields of his soldiers, and so to proceed to battle. He did as he had been commanded, and he marked on their shields the letter X, with a perpendicular line drawn through it and turned round thus at the top (P), being the cipher of CHRISTOS. Having this sign, his troops stood to arms.Lactantius' account is decidedly more observational and less explanatory than that of Eusebius. He offers no clear rationale for Constantine's decision to follow the instructions given to him by God. Perhaps Constantine experienced a genuine conversion after seeing God in his dream. Or maybe he followed God's instructions not out of faith but out of desperation for a winning edge during the battle. Lactantius leaves the decision to the reader.
Constantine and the Conversion of Europe (A.H.M. Jones)