During a war against his brother-in-law and co-emperor, Maxentius. According to the historian Eusebius. Constantine was convinced that he needed divine assistance. While he was praying for such assistance, God sent him a vision of a cross of light at midday, bearing the inscription "in hoc signo vinces " ("in this sign you will be victorious"). That night he had a dream that reaffirmed his earlier vision. God told him to use the sign he had been given as a safeguard in all of his battles. Thus, Constantine converted to Christianity and ordered the symbol of his Savior's name (the intersection of the Greek letter chi and rho) to represent his army