Commentary Purg IX 117-126

Sensible allegorical expositions of the two keys are found variously, but of particular use is Poletto's gloss (DDP Poletto.Purg.IX.115-117).  The golden key denotes the authority to absolve granted by Christ directly; the silver, the judgment necessary in the priest to be sure the penitent is truly deserving of absolution.  Poletto cites passages in St. Thomas to show Dante's closeness to them in this part of his description of the process of entering purgatory (Summa Th., III, Suppl., qq. 17-20).  Once the priest has judged the penitent ready for absolution (using his silver key), he then uses the golden one to complete the opening of the door.  The priest, of course, may err in wanting to allow an unfit soul to enter; in that case the golden key will not turn in the lock -- but even so, God is disposed to err on the side of mercy and will overrule a prelate who is niggardly in pardoning.

      The fact that a priest may err in his judgment makes it disturbing that this figure is presented as being literally an angel (who thus should be free of such weak discernment).  It would seem more logical if we dealt here with an allegorical figure, Priestliness, the Petrine warder of the gate, a composite figure representing a class, not a particular historical being.  And, indeed, the angel does not behave in any other way.