Immediately before a crucial battle with his rival Maxentius in AD 312, Constantine saw the Greek letters chi and rho in a dream and heard a voice say “In Hoc Signo Vinces” (“conquer by this sign” in Latin). Chi and rho are the first two letters in the Greek word for Christ, Christos. Constantine ordered his men to affix this symbol to their shields, and he defeated Maxentius, transforming a military victory into a spiritual triumph for Christ. According to Christian tradition, it was this battle that converted the Roman principate into a Christian empire.
It is not entirely clear, however, that Constantine believed he was visited by a Christian vision: the triumphal arch erected in the Coliseum to commemorate the battle only reads “through the prompting of the deity.” Constantine was certainly friendly to Catholicism: he convened the Council of Nicea in 325 and commissioned the creation of new copies of the Bible in 331. However, the official religion of Constantine’s empire was pagan sun worship, not Christianity, and Constantine’s courtship of the Christians, who were by the time of his reign extremely numerous, may have been simple political expediency. Constantine’s deathbed conversion to Christianity in 337 is also considered to be highly suspect. Whatever the case, Catholic authorities, who had been persecuted for so long, used the occasion to seize power and persecute paganism and unorthodox forms of Christianity.