This article intends to prove that the first constitution of the IVth Lateran Council, Firmiter, is a direct source for Peire de Corbian’s Tesaur. This allows establishing a more sicure terminus post quem for this Provençal enyclopaedic poem (1215). The references in question affirm the trinitarian faith in One God, Creator ex nihilo of the material and the immaterial world and hence oppose contrary Cathar teachings. As is shown by the character of the manuscript tradition, this intentional antiheretic bias of the text was determining for the poem’s fortune. Still, the simultaneous presence of the idea of man as a remplacement for fallen angels demonstrates that Peire did not rely consistently on the Council’s promulgations: rejected opinions could persist in generally orthodox texts.