In the "Life of Saint Cyril-Constantine the Philosopher",
Rastislav, the duke of Moravia sent an embassy to Constantinople asking
Emperor Michael III to send learned men to the Slavs of
Great Moravia, who being already baptised, wished to have the liturgy in their own language, and not Latin and Greek. Emperor called for
Constantine and asked him if he would do this task, even though being in poor health. Constantine replied that he would gladly travel to Great Moravia and teach them, as long as the Slavs had their own alphabet to write their own language in, to which the Emperor replied that
not even his grandfather and father and let alone he could find any evidence of such an alphabet. Constantine was distraught, and was worried that if he invents an alphabet for them he'll be labelled a heretic.
Even if some form of writing existed among the Slavs in previous centuries, by the 9th century the learned men in the
Eastern Roman Empire were not aware of its existence in any of the Slavic lands that they had sent missionaries or ambassadors to. Either this writing had died out or it wasn't a real form of writing, but rather just "tallies and sketches" as mentioned in Chernorizets Hrabar's
An Account Of Letters, using which books could not be written.