What is public preaching of heresy?

Canon 15 does not trigger on rumor, suspicion, a private opinion, or a single slip of the tongue. Its words are precise: the bishop must preach the heresy publicly (δημοσίᾳ) and teach it “bareheaded” (γυμνῇ τῇ κεφαλῇ) in the Church, openly, unmistakably, as teaching.

The obligatory school adds a sober observation: teaching is done by deeds as well as words. A hierarch who concelebrates or prays publicly and repeatedly with those the canons forbid is proclaiming a doctrine of communion as loudly as any sermon, the eye, they note, is a surer witness than the ear. But the standard remains high either way: the matter must be certain, public, and persistent, or the one who withdraws risks the unjust judgment Lesson 11 warns about.

• δημοσίᾳ, publicly, before the church, not in private conversation.
• γυμνῇ τῇ κεφαλῇ, “with bare head”: uncovered, undisguised, unmistakable.
• Admonition, the apostolic pattern of first and second warning (Titus 3:10) that precedes rejection.

Canon 15's own wording; Titus 3:10 on admonition before rejection; the Trikamenas study (catalogued in the Library) on heresy shown in deeds.

Nestorius preached against the Theotokos from the ambo of the capital, the textbook case of “bareheaded” teaching. By contrast, the Fathers did not wall off from bishops for private errors later corrected.

“Any error I detect in my bishop licenses separation.” No: the canon requires an already condemned heresy (Lesson 4), publicly and persistently taught. Everything short of that is handled by admonition, protest, and the Church's ordinary processes.

• Canon 15, key terms
• Lesson 11: the dangers of private judgment