“LET US, therefore, not be ashamed of the cross of Christ; but though another hide it, do thou openly seal it upon thy forehead, that the devils may behold the royal sign and flee trembling far away. Make then this sign at eating and drinking, at sitting, at lying down, at rising up, at speaking, at walking: in a word, at every act.”
~St. Cyril of Jerusalem (c. 313 – 386 AD): Catechetical Discourses, 4, 14.