Even before Christianity swept through the Roman Empire, the roots of Ash Wednesday were present. Roman generals would be honored after military victories with “triumphs,” which were a combination parade/religious ceremony. But a slave would ride alongside the general, whispering in his ear, “Remember you are mortal” to keep his ego in check. You can read more in Tertullian’s Apology, chapter 33: http://www.logoslibrary.org/tertullian/apology/33.html