 Maxentius organized his forces — still twice the size of Constantine 's — in long lines facing the battle plain , with their backs to the river . Constantine 's army arrived at the field bearing unfamiliar symbols on either its standards or its soldiers ' shields . According to Lactantius , Constantine was visited by a dream the night before the battle , wherein he was advised " to mark the heavenly sign of God on the shields of his soldiers ... by means of a slanted letter X with the top of its head bent round , he marked Christ on their shields . " Eusebius describes another version , where , while marching at midday , " he saw with his own eyes in the heavens a trophy of the cross arising from the light of the sun , carrying the message , In Hoc Signo Vinces or " with this sign , you will conquer " ; in Eusebius 's account , Constantine had a dream the following night , in which Christ appeared with the same heavenly sign , and told him to make a standard , the labarum , for his army in that form . Eusebius is vague about when and where these events took place , but it enters his narrative before the war against Maxentius begins . Eusebius describes the sign as Chi ( Χ ) traversed by Rho ( Ρ ) : ☧ , a symbol representing the first two letters of the Greek spelling of the word Christos or Christ . In 315 AD a medallion was issued at Ticinum showing Constantine wearing a helmet emblazoned with the Chi Rho , and coins issued at Siscia in 317 / 318 AD repeat the image . The figure was otherwise rare , however , and is uncommon in imperial iconography and propaganda before the 320s . 
