Constantine triumphs with the sign of Christ at the Mulvian Bridge - 312 AD

Constantine, Emperor of a divided Rome faces his rival Maxentius who possesses a larger army at the Mulvian bridge over the Tiber river. Constantine had a dream in which he saw the first to letters of "Christ" (Chi Rho) with the words "By this sign you will conquer". Constantine put the Chi Rho on the shields of his men and was victorious.
(Ancient Roads - From Christ to Constantine Part 5 Pagans and the Cult of Martyrs - 2015)