The Form of Prayer given by Constantine to his Soldiers
“We acknowledge you the only God: we own you, as our King and implore your succor. By your favor have we gotten the victory: through you are we mightier than our enemies. We render thanks for your past benefits, and trust you for future blessings. Together we pray to you, and beseech you long to preserve to us, safe and triumphant, our emperor Constantine and his pious sons.” Such was the duty to be performed on Sunday by his troops, and such the prayer they were instructed to offer up to God.
Source: Life of Constantine (New Advent)