What does Apoteichisis mean?

Apoteichisis comes from the Greek ἀπο- (“off, away”) and τεῖχος (“a wall”): to wall oneself off. In the Church's use it means withdrawing communion from your own bishop, ceasing to name him in the services, when he openly teaches a heresy the Church has already condemned. It is a defensive act: a wall raised against error, not a doorway into a new church.

• ἀποτείχισις, walling off; the withdrawal of communion described in this course.
• Commemoration, naming one's bishop aloud in the Liturgy, the audible sign of being in communion with him.
• Communion, here, ecclesiastical communion: shared liturgical life, not only receiving the Eucharist.

The term's canonical home is Canon 15 of the First and Second Council (861), which protects those who withdraw from a bishop “publicly preaching” a “condemned” heresy “before synodical judgment.” Lesson 5 examines the text.

During the Iconoclast crisis, confessors such as St Theodore the Studite withdrew from bishops who taught the condemned error, without founding a rival church, the pattern the canon later describes.

Apoteichisis is often confused with schism or with joining a breakaway group. In its canonical form it is neither: it is a temporary, defensive withdrawal that regards itself as remaining in the Church and awaiting her judgment. When a separation instead sets up its own hierarchy and declares everyone else graceless, it has become a different thing, the subject of Lesson 12.

• What Is Apoteichisis?, the fuller introduction
• Canon 15, text and interpretations