He orders the Sign of the Saviour's Cross to be engraven on his Soldiers' Shields.
Life of Constantine, with Orations · Eusebius of Caesarea
EusVC 3.219.1
And not only so, but he also caused the sign of the salutary trophy to be impressed on the very shields of his soldiers; and commanded that his embattled forces should be preceded in their march, not by golden images, as heretofore, but only by the standard of the cross.