Read "Feast of St. Anthony" at The Eldritch Dark:
As Omoultakos pointed out in my blog post on "The Sciapod", Clark Ashton Smith (CAS) was an admirer of the French writer Gustave Flaubert, and one can assume that the haiku "Feast of St. Anthony" was inspired by Flaubert's novel La Tentation de Saint Antoine (1874), known in English as The Temptation of Saint Anthony.
The poem transforms the meaning of the word "feast" as it is usually understood in the context of the calendar of saints into the more familiar notion of a banquet, but with the addition of birds from myth and legend as guests at the feast. This seems to incorporate one of the famous temptations of St. Anthony, when he was assailed by demons in the guise of wild beasts.
It's an unusual topic for CAS, given the explicitly Christian subject matter, but as Omoultakos noted, likely more a consequence of CAS' interest in the writings of Flaubert rather than an expression of religious faith.