The auto sacramental began, on the most widely repeated account, as a moving picture rather than a play. Medieval Corpus Christi processions in Spain carried decorated floats and tableaux vivants through city streets, silent scenes meant to be looked at rather than watched as drama. Over the fifteenth and early sixteenth centuries those tableaux gained speech, then dialogue, then plot, until they became a self standing form performed on wagons, carros, wheeled between a city's squares so a single production reached several audiences in one feast day.
Gil Vicente's Auto de San Martin, staged at the Corpus Christi procession in Lisbon in 1504, is the work most often named as the first true auto sacramental, and the date is repeated confidently in tertiary sources. It is also, on a closer reading, a doubtful claim: the play has no allegorical characters of the kind that came to define the genre and does not itself dramatize the Eucharist, the very feature the term names. Historians who make this point trace the form's real roots further back, to the plain devotional tableaux documented in Spain from the thirteenth century, well before any surviving text gives the genre the allegorical, doctrine arguing shape it would later carry.
Whatever its true beginning, the mature form is unmistakable. Its characters are personifications rather than named people: Faith, the Church, the World, Sin, arguing out a point of Catholic teaching on the Eucharist through allegory rather than biography. Municipal councils across Spain and its American colonies commissioned and funded productions each Corpus Christi, and the genre grew steadily more elaborate across the sixteenth and seventeenth centuries, right up to the moment a single playwright came to own it almost entirely.