)αχιλλεὺς ἐρέει )αθήνῃ, τὸ δὲ τετελεσμένον ἔσται. ὀίω ταῦτα τελέεσθαι.
In stilted English I think this means;
“Achilles will say to Athena, but that what has been fulfilled will be. I think these things will be fulfilled.”
Is this an idiomatic was of saying; “What ever will be will be, so let it be”?
It is not “but”, but “and” that will be fulfilled. It is a way to emphasize the weight, the earnestness of the words expressed (Iliad, I, 212). So is also the following sentence, which has been taken from Iliad, I, 204. Note that the meaning of the first sentence (words of the goddess Hera who wants to calm down Achilles) is stronger that the meaning of the second one (words of Achilles who says that he will kill Agamemnon).
Because of the perfect tense compared to the future?