Trilingual heresy

Trilingualism was rejected in the 850s by Saints Cyril and Methodius, Byzantine brothers and missionaries who introduced a Christian liturgy in the vernacular of their Slavic converts, a language now called Old Church Slavonic.

[1] It appears as a neologism in several chapters of a contemporary hagiography of Cyril (then named Constantine), most prominently when recounting a disputation in Venice in AD 867 while he and Methodius were en route to the Holy See, bringing relics of Pope Clement I and hoping to resolve a jurisdictional dispute in Great Moravia with Latin Rite missionaries sent by the Bishop of Salzburg.

[3] In St. Mark's Square, hostile clerics (branded "Latin accomplices" of the devil[1]) "assembled against [Constantine] like ravens against a falcon and raised the trilingual heresy".

[3] A generation later, Chernorizets Hrabar's defence of the Glagolitic script used to write Old Church Slavonic, likewise, deprecates trilingualism on the basis that the Slavs would never have been converted if their own language had not been used.

Some historians regard trilingualism as a straw man invented by Orthodox supporters of autocephaly or national churches, but never actually promoted by the Papacy or Constantinople.