This objection is the result of playing “the telephone game,” where a message becomes distorted as it gets repeated.
You have contributed to this distortion as well, although I assume you did not intend to. Let’s start with your claim and work backwards.
“Gilbert gives a quote from Augustine.” In fact, Gilbert does not give a quote from Augustine. He quotes one F. Graf, who was writing about Augustine. Graf cites Augustine’s work City of God, book 10, chapter 21, and claims that here Augustine lays the"theoretical foundation for the cult of the saints." Gilbert takes Graf’s claim as fact and runs with it.
Here is the relevant text of the chapter of City of God which Graf is referencing:
" CHAPTER 21 – OF THE POWER DELEGATED TO DEMONS FOR THE TRIAL AND
GLORIFICATION OF THE SAINTS, WHO CONQUER NOT BY PROPITIATING THE
SPIRITS OF THE AIR, BUT BY ABIDING IN GOD.
The power delegated to the demons at certain appointed and well-adjusted seasons, that they may give
expression to their hostility to the city of God by stirring up against it the men who are under their
influence, and may not only receive sacrifice from those who willingly offer it, but may also extort it
from the unwilling by violent persecution; – this power is found to be not merely harmless, but even
useful to the Church, completing as it does the number of martyrs, whom the city of God esteems as all
the more illustrious and honored citizens, because they have striven even to blood against the sin of
impiety. If the ordinary language of the Church allowed it, we might more elegantly call these men our
heroes. For this name is said to be derived from Juno, who in Greek is called Here, and hence, according
to the Greek myths, one of her sons was called Heros. And these fables mystically signified that Juno
was mistress of the air, which they suppose to be inhabited by the demons and the heroes, understanding
by heroes the souls of the well-deserving dead. But for a quite opposite reason would we call our martyrs
heroes – supposing, as I said, that the usage of ecclesiastical language would admit of it – not because
they lived along with the demons in the air, but because they conquered these demons or powers of the
air, and among them Juno herself, be she what she may, not unsuitably represented, as she commonly is
by the poets, as hostile to virtue, and jealous of men of mark aspiring to the heavens.
Virgil, however, unhappily gives way, and yields to her; for, though he represents her as saying, “I am
conquered by Æneas,” Helenus gives. Æneas himself this religious advice: “Pay vows to Juno: overbear
Her queenly soul with gift and prayer.”
In short, I would say that Mr. Gilbert should examine the source for his claims, and then move on. It doesn’t really warrant any other response.