Is Apoteichisis a schism?

St Basil's First Canonical Epistle gives the Church her taxonomy: heresy (a difference in the faith itself), schism (a curable separation over ecclesiastical questions), and parasynagogue (insubordinate assemblies). Where in this map does walling off fall?

The canon answers for itself: those who withdraw in its one protected case “have not fragmented the Church's unity by schism, but have been eager to deliver her from schisms.” On the canon's own logic the schismatic is the pseudo bishop who tore the Church from the faith; the one who walls off is sealing the tear. But the acquittal is conditional, it holds only while the withdrawal keeps the canon's shape: temporary, defensive, within the Church, awaiting her judgment. Lesson 12 marks the line where that shape is lost.

• Schism (σχίσμα), in Basil's sense, a separation over curable ecclesiastical questions, and, in common use, any rupture of the Church's unity.
• Parasynagogue, an unlawful rival assembly, what walling off must never become.

St Basil, Letter 188, canon 1; Canon 15's acquittal clause; the canonized confessors (Maximus, Theodore, Mark) whom the Church honours rather than condemns.

St Euthymius the Great refused the communion of the Monophysite usurper of Jerusalem and withdrew to the deep desert, and returned to communion the moment the lawful patriarch was restored. Temporary, defensive, within: the shape the canon protects.

“All separation is schism.” If that were so, the Church would have to condemn her own confessors. The canons distinguish the rupture that abandons the faith from the withdrawal that guards it, and judge them oppositely.

• Does stopping commemoration create schism?
• Heresies & confessions, Basil's taxonomy