Gregory I, Pope. [Letter to Theodelind, queen of the Lombards, 594] in Gregorii I papae Registrum epistolarum 4.33, 1.268-69

[Letter from Pope Gregory I to Theodelind, queen of the Lombards]
Apparently the first letter () did not reach Theodelind, so Gregory repeats his warning and specifies the four councils or synods that are accepted.
Online at Epistolae (See also Bibliographia record).