That he directed even his Pagan Soldiers to pray on the Lord's Day
With regard to those who were as yet ignorant of divine truth, he provided by a second statute that they should appear on each Lord's day on an open plain near the city, and there, at a given signal, offer to God with one accord a prayer which they had previously learned. He admonished them that their confidence should not rest in their spears, or armor, or bodily strength, but that they should acknowledge the supreme God as the giver of every good, and of victory itself; to whom they were bound to offer their prayers with due regularity, uplifting their hands toward heaven, and raising their mental vision higher still to the king of heaven, on whom they should call as the Author of victory, their Preserver, Guardian, and Helper. The emperor himself prescribed the prayer to be used by all his troops, commanding them, to pronounce the following words in the Latin tongue:
Source: Life of Constantine (New Advent)