“not writing it out on paper, but engraving it by the memory upon your heart”

not writing it out on paper, but engraving it by the memory upon your heart

Cyril of Jerusalem · The Catechetical Lectures · NPNF2 VII, Catechetical Lectures, Lecture V, 12

How the creed was handed over before anyone thought to print it: recited, memorised, never written down, and not to be said where a catechumen might overhear. The discipline of the secret in one sentence, from the man administering it.