After the saint's death in 885, Constantine was jailed by the Germanic clergy in Great Moravia and sold as slave in Venice.
His most significant literary work was Учително евангелие (The Didactic Gospel), usually dated to the first years of the reign of Bulgarian tsar Simeon I, 893 – 894.
The compilation also features the poetic preface Азбучна молитва (Alphabet Prayer), the first original poetry in Old Church Slavonic.
[1] In 906, by commission from Simeon I, the author translated Четири слова против арианите (Four Epistles against the Arians) by Saint Athanasius of Alexandria, as a response to the beginning of the spread of heresies in medieval Bulgaria.
Constantine is also the alleged author of Служба на Методия (Service for Methodius), in which he relates the struggle of Saint Methodius for the recognition of Old Church Slavonic, as well as of Проглас към евангелието (Proclamation of the Holy Gospels) in which he rejects and admonishes the admiration of the foreign language (mean.