Augustine—The City of God Against the Pagans
II Of the Falseness of the history which ascribes many thousands of years to times gone by
“Let us, then, omit the conjectures of men who know not what they say, when they speak of the nature and origin of the human race. … They are deceived, too, by those highly mendacious documents which profess to give the history of many thousand years, though, reckoning by the sacred writings, we find that not 6000 years have yet passed.”
You left out the entire context of Augustine’s argument. He was arguing against the “highly mendacious documents” of the pagans which purported to give a history of the human race of many thousands of years, and none of which agreed with each other.
But be that as it may, Augustine is arguing there about the date *of the creation of the human race*—NOT the creation of the world, which as you know as two separate things, especially if one holds some variant of the day-age hypothesis.
And while I’m on the subject, Augustine didn’t believe the universe was created in six days either. He believed it was all created SIMUL = “simultaneously” and that the 6 days in Genesis represent progressive revelations to the angels. ANd he ALSO said in his “On the Literal Interpretation of Genesis” that it was an extraordinarily difficult text to interpret and that perhaps a better explanation could be found than that which he was offering.
He was not reckless with the text. He was careful. So his admonition stands against those who prop up one explanation and pretend to speak for all of Christianity in this regard instead of being a little more humble and admitting that we don’t have all the answers.