Trilingual heresy

In Slavic Christianity, the trilingual heresy or Pilatian heresy (less pejoratively trilingualism) is the idea that Biblical Hebrew, Greek, and Latin are the only valid liturgical languages or languages in which one may praise God. 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.