The Slavonic tradition produced abecedarian poems, compositions in which each successive line opens with the next letter of the alphabet in order. The best known is the Alphabet Prayer, a verse preface written to stand in front of a Gospel translation, whose first line begins with azu. Its authorship is argued. The attribution most often made is to Constantine of Preslav, a pupil of the Thessalonican brothers working in the Bulgarian capital, but an older tradition assigns it to Cyril himself, and the poem survives only in copies made long after either man was dead. Nothing in the surviving manuscripts settles the question.
The form matters for a reason beyond its literary interest. An acrostic is a lock on a sequence. If a poem has one line per letter and the lines are in order, then the poem records what its author believed the order of the alphabet to be, and it records how many letters there were. Where the manuscript tradition of the alphabet itself is thin, and for the earliest period it is very thin indeed, an abecedarian poem is a witness that cannot be quietly reordered by a later copyist without the damage showing.
Acrostics of this kind are not a Slavonic invention. They run through the Hebrew psalms and through Byzantine hymnography, and a translator trained in Constantinople would have known them as a standard device. Writing one in a new alphabet is a way of declaring that the new letters are a complete and ordered set, fit to carry scripture, which is precisely the claim the mission needed to make.