According to Catholics, what is the source of the Trinitarian doctrine of the Athanasian Creed, tradition or divine inspiration?
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.
There are many roles of the Holy Ghost in the Church. The roles at play here are that He
1. guards the deposit of faith or Revelation in all its truth,
2. prevents the Church from teaching error,
3. guides the development of doctrine,
4. gives council to the church (and all His other gifts, too),
5. enables the Church to interpret the true sense of Scripture.