Saints Cyril and Methodius were "the Brothers from Thessaloniki" who invented an alphabet for the Slavs and translated, written in their alphabet "Glagolitsa" the Holy Scripts in the old Bulgarian language (that is also called old Slavonic and old Church Slavonic language).
They were real people. Here is the Pater noster prayer as (most probable) translated by them. Cyril (also known as Constantine) the Philosopher died in Rome and his sarcophagus is now in the Vatican. The church is from 1897 and was sponsored by the grand father of Beba, a coed of mine from the time I went to the nearby 13th school for general education of Sofia back 1960-64. The 24th of May is the National Day of the Bulgarian Literacy and Culture (according to the date for the old church fest of Cyril and Methodius from the time the Bulgarian Church used the Gregorian Calendar. Now the day is at May 11th, but the Peoples Fest, as pronounced some years after the Communist came in 1944, rests at May 24th, that is to-morrow.) Original public domain image from Wikimedia Commons