Sankt Gallen, Stiftsbibliothek 742, fol. 272r

Italian ca. 1280, Gregory IX, Decretales

A canon from the Council of Poitiers in 1079 banning clergy from bearing arms became part of the normative law of the church by the late twelfth century.

Council of Poitiers 1079

1 Comp. 3.1.2 (X 3.1.2): 

Clerici arma portantes et usurarii

excommunicentur.

(Clerics who bear arms or who are usurers may be excommunicated).

Lawrence G. Duggan, Armsbearing and the Clergy in the History and Canon Law of Western Christianity (Boydell & Brewer, 2013) 99-100, 136.