This study presents the doctrinal environment of the Nicaeno-Constantinopolitanum, including its lost Tomus, mentioned by the synodal epistle of 382, in light of which the Creed’s theology ought to be explained. Despite some lacunae, modern scholarship established links between the West (Rome), the Antiochene council of 379 and the ecumenical council of 381. The Fathers’ attempts to find new methods of expressing a pneumatology based on the threefold ὁμοούσια demonstrate that the consubstantiality was meant to be extended to the Spirit. The Early Church regarded the Nicene Creed as being “the faith” (ἡ πίστις) or “the symbol” (τὸ σύμβολον). The other three formulae (of 381, 433 and 451) were definitions or explanations (ὅροι) of, yet by no means additions to “the ancient faith of the 318 holy Fathers”. This, of course, does not mean that these four credal statements should not be regarded as being of equal rank today, since they constitute the indispensable basis for any ecumenical discussion.