Two wise and lovely paragraphs with a masterly final sentence.
There is knowledge to be had, after all, about the earth, about the sky, about the other elements of this world, about the movements and revolutions or even the magnitude and distances of the constellations, about the predictable eclipses of moon and sun, about the cycles of years and seasons, about the nature of animals, fruits, stones, and everything else of this kind. And it frequently happens that even non-Christians will have knowledge of this sort in a way that they can substantiate with scientific arguments or experiments. Now it is quite disgraceful and disastrous, something to be on one's guard against at all costs, that they should ever hear Christians spouting what they claim our Christian literature has to say on these topics, and talking such nonsense that they can scarcely contain their laughter when they see them to be toto caelo, as the saying goes, wide of the mark. And what is so vexing is not that misguided people should be laughed at, as that our authors should be assumed by outsiders to have held such views and, to the great detriment of those about whose salvation we are so concerned, should be written off and consigned to the waste paper basket as so many ignoramuses.
Whenever, you see, they catch out some members of the Christian community making mistakes on a subject which they know inside out, and defending their hollow opinions on the authority of our books, on what grounds are they going to trust those books on the resurrection of the dead and the hope of eternal life and the kingdom of heaven, when they suppose they include any number of mistakes and fallacies on matters which they themselves have been able to master either by experiment or by the surest of calculations? It is impossible to say what trouble and grief such rash, self-assured know-alls cause the more cautious and experienced brothers and sisters. Whenever they find themselves challenged and taken to task for some shaky and false theory of theirs by people who do not recognize the authority of our books, they try to defend what they have aired with the most frivolous temerity and patent falsehood by bringing forward these same sacred books to justify it. Or they even quote from memory many things said in them which they imagine will provide them with valid evidence, not understanding either what they are saying, or the matters in which they are asserting themselves (1 Tim 1:7).
From The Literal Meaning of Genesis, Book 1, par. 19 (39) (trans. Hill). Counsel to be taken to heart. By the way, Augustine experienced precisely this sort of thing when he was a Manichee. See Confessions, Book V.