I don't buy a lot of that, at least if we're referring to the 18th century.

• The founders of America knew damn well that there were no such things as gods, at least not ones that actively intervened in any way we could detect.

• They were wrong about some details of astronomy, but they had most of the basic outlines right (Lagrange's works describe the celestial mechanics of the solar system in quite some detail).

• The theories of classical mechanics were known and well understood. Quantum mechanics and relativity weren't, of course, but I am hesitant to refer to this as people being wrong, as there were very few observations available to them which required these to be explained (the perihelion advance of Mercury, for instance, wasn't discovered until 1859).

• The 18th century view of cosmology was essentially ours, except that it lacked knowledge about how it was organized on a larger scale (galaxies within clusters within superclusters and all that) due to the lack of sufficiently powerful telescopes, and many supposed the universe to be infinite instead of beginning with the Big Bang.

• The structure of democratic government invented during this period works pretty darn well, by comparison with everything that came before. There have, for instance, been no wars in Western Europe for sixty years, something that has never happened before.

• Lavoisier and Lomonosov's theories of chemistry were, in fact, largely correct. The periodic table wasn't known, but there was no widely used wrong system of grouping the elements.

• The full theory of evolution was not known (people still believed in spontaneous generation, for instance), but the idea that groups of similar species arose from a common ancestor by descent with modification was widely known and accepted.

The proper extrapolation from this is not "everything you know is wrong", but "there are lots of things you don't know, and lots of non-technical things you 'know' are wrong."