St Justin Martyr on the Ubiquity of the Cross

And this, as the prophet foretold, is the greatest symbol of His power and role; as is also proved by the things which fall under our observation. For consider all the things in the world, whether without this form they could be administered or have any community. For the sea is not traversed except that trophy which is called a sail abide safe in the ship; and the earth is not ploughed without it: diggers and mechanics do not their work, except with tools which have this shape. And the human form differs from that of the irrational animals in nothing else than in its being erect and having the hands extended, and having on the face extending from the forehead what is called the nose, through which there is respiration for the living creature; and this shows no other form than that of the cross. And so it was said by the prophet, "The breath before our face is the Lord Christ." And the power of this form is shown by your own symbols on what are called "vexilla" [banners] and trophies, with which all your state possessions are made, using these as the insignia of your power and government, even though you do so unwittingly. And with this form you consecrate the images of your emperors when they die, and you name them gods by inscriptions. Since, therefore, we have urged you both by reason and by an evident form, and to the utmost of our ability, we know that now we are blameless even though you disbelieve; for our part is done and finished.