Michael III in the context of "Photian schism"

Iconoclasm (from Ancient Greek εἰκών (eikṓn) 'figure, icon' and κλάω (kláō) 'to break') is the belief in the importance of the destruction of icons and other images or monuments, often for religious or political reasons. Those who engage in or support iconoclasm are called iconoclasts, a term that has come to be applied figuratively and more broadly to anyone who challenges "cherished beliefs or venerated institutions on the grounds that they are erroneous or pernicious".

Conversely, one who reveres or venerates religious images is called (by iconoclasts) an iconolater; in a Byzantine context, such a person is called an iconodule or iconophile. Iconoclasm does not generally encompass the destruction of the images of a specific ruler after their death or overthrow, a practice better known as damnatio memoriae.

The Glagolitic script (/ˌɡlæɡəˈlɪtɪk/ GLAG-ə-LIT-ik, ⰳⰾⰰⰳⱁⰾⰻⱌⰰ, glagolitsa) is the oldest known Slavic alphabet. It is generally agreed that it was created in the 9th century for the purpose of translating liturgical texts into Old Church Slavonic by Saint Cyril, a monk from Thessalonica. He and his brother Saint Methodius were sent by the Byzantine Emperor Michael III in 863 to Great Moravia after an invitation from Rastislav of Moravia to spread Christianity there. After the deaths of Cyril and Methodius, their disciples were expelled from Moravia, and they moved to the First Bulgarian Empire instead. The Early Cyrillic alphabet, which was developed gradually in the Preslav Literary School by scribes who incorporated some Glagolitic letters when writing in the Greek alphabet, gradually replaced Glagolitic in that region. Glagolitic remained in use alongside the Latin script in the Kingdom of Croatia and alongside Cyrillic until the 14th century in the Second Bulgarian Empire and the Serbian Empire; in later periods, it was used mainly for cryptographic purposes.

Glagolitic also spread to the Kievan Rus' and the Kingdom of Bohemia. Although its use declined there in the 12th century, some manuscripts in the territory of the former retained Glagolitic inclusions for centuries. It had also spread to Duklja and Zachlumia in the Western Balkans, from where it reached the March of Verona. There, the Investiture Controversy afforded it refuge from the opposition of Latinizing prelates and allowed it to entrench itself in Istria, from which place it spread to nearby lands. It survived there and as far south as Dalmatia without interruption into the 20th century for Church Slavonic in addition to its use as a secular script in parts of its range, which at times extended into Bosnia, Slavonia, and Carniola, in addition to 14th–15th century exclaves in Prague and Kraków, and a 16th-century exclave in Putna.

The Feast of Orthodoxy (or Sunday of Orthodoxy or Triumph of Orthodoxy) is celebrated on the first Sunday of Great Lent in the Eastern Orthodox Church and other churches using the Byzantine Rite to commemorate, originally, only the final defeat of iconoclasm on the first Sunday of Lent in 843, and later also opposition to all heterodoxy.

Theodora (Greek: Θεοδώρα; c. 815 – c. 867), sometimes called Theodora the Armenian or Theodora the Blessed, was Byzantine empress as the wife of Byzantine emperor Theophilos from 830 to 842 and regent for the couple's young son Michael III, after the death of Theophilos, from 842 to 856. Theodora is most famous for bringing an end to the second Byzantine Iconoclasm (814–843), an act for which she is recognized as a saint in the Eastern Orthodox Church. Though her rule saw the loss of most of Sicily and failure to retake Crete, Theodora's foreign policy was otherwise highly successful; by 856, the Byzantine Empire had gained the upper hand over both the Bulgarian Empire and the Abbasid Caliphate, and the Slavic tribes in the Peloponnese had been forced to pay tribute, all without decreasing the imperial gold reserve.

Possibly of Armenian descent, Theodora was born into a rural family of traders and military officials in Paphlagonia. In 830 she was selected by Euphrosyne, step-mother of the emperor Theophilos, as a candidate in a bride-show for the young emperor. After being chosen by Theophilos she was crowned empress on 5 June 830. Theophilos was an iconoclast, who opposed the veneration of icons; it is unclear how aware he was that Theodora was an iconophile. The couple had seven children and Theodora was a loyal participant in imperial affairs and ceremonies, but she continued to secretly venerate icons throughout her husband's reign. Theophilos died of dysentery on 20 January 842, probably younger than 30. On his deathbed, he named Theodora as regent for their two-year-old son Michael III and designated a selection of advisors to assist her. The most prominent of these advisors was the logothete and eunuch Theoktistos, who would become a close confidant of Theodora.

Basil I, nicknamed "the Macedonian" (Greek: Βασίλειος ὁ Μακεδών, romanized: Basíleios ō Makedṓn; 811 – 29 August 886), was Byzantine emperor from 867 to 886. Born to a peasant family in Macedonia, he rose to prominence in the imperial court after gaining the favour of Emperor Michael III, whose mistress he married on his emperor's orders. In 866, Michael proclaimed him co-emperor. Fearing a loss of influence, Basil orchestrated Michael's assassination the next year and installed himself as sole ruler of the empire. He was the first ruler of the Macedonian dynasty.

Despite his humble origins, Basil was an effective and respected monarch. He initiated a complete overhaul of Byzantine law, an effort continued by his successor that ultimately became the Basilika. On the foreign front, he achieved military success against the heretical Paulicians, whom he subjugated in 872. He also pursued an active policy in the west, allying with Carolingian emperor Louis II against the Arabs, which led to a new period of Byzantine domination in Italy. Upon his death in a hunting accident in 886, he was succeeded by his son Leo VI, also rumoured to have been the son of Michael III.

Rastislav or Rostislav (Latin: Rastiz; Greek: Ῥασισθλάβος, romanized: Rhasisthlábos) was the second known ruler of Moravia (846–870). Although he started his reign as vassal to Louis the German, the king of East Francia, he consolidated his rule to the extent that after 855 he was able to repel a series of Frankish attacks. Upon his initiative, brothers Cyril and Methodius, sent by the Byzantine Emperor Michael III in 863, translated the most important Christian liturgical books into Slavonic. Rastislav was dethroned by his nephew Svatopluk I of Moravia, who handed him over to the Franks. He was canonized by the Eastern Orthodox Church in 1994 and is also known as Saint Rastislav.