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Discussing who has the right to propose a symbol of faith (article of a creed), St. Thomas Aquinas explains the Athanasian Creed in Summa Theologica II-II q. 1 a. 10 ad 3:
Athanasius drew up a declaration of faith […] by way of an exposition of doctrine, as appears from his way of speaking. But since it contained briefly the whole truth of faith, it was accepted by the authority of the Sovereign Pontiff, so as to be considered as a rule of faith.