The scientism "faith" of Modernity rose in panicked reaction to the late 19th century's many scientific discoveries revealing unexpected complexities in Deep reality.
Earlier late 19th century modernization (a unconscious process rather than a self conscious faith like Modernity) had already brought home the complexity of Surface reality to the educated middle class of the largest imperial civilizations.
That emotionally overwhelming complexity had arrived via half tone print media illustrations at their breakfast table or via the sight, sounds and smell of all the products of their imperial colonies (& sometimes even their imperial subjects themselves) hourly disembarking off steamships at their empire's metropolitan centre.
Repulsed by the facts of surface complexity revealed by Natural History, the science-minded middle class had sought mental relief in the arms of Natural Philosophy.
For millenniums the natural philosophers had claimed, with never any facts to support their theories, that ultimately reality was extremely simple and very stable: easily knowable,controllable and predictable.
Armed now with far more precise scientific instrumentation, the Natural philosophy end of Science, led by theoretical physicists and their fellow travellers in other sciences, were finally ready to put their funders' grant money where the philosophers' mouth was.
Problem : the instruments soon revealed that Deep reality was far far more complex than even Surface reality.
Most of the discoverers, despite generally regarding themselves as Positivists, refused (even into their graves) to fully accept the intellectual consequences of the results revealed before their own eyes.
Max Planck (quantum mechanics), Albert Einstein (relativity) and Frederick Griffith (HGT DNA) are merely a few examples of this intellectual discomfort.
While mainstream Science struggled to bury and explain away this uncomfortable evidence of Deep complexity, mainstream middle class civilizations responded by dividing all reality into just two general categories : worthy/simple/purity and unworthy/complex/ dirt.
Actually 'purity' and 'dirt' didn't have a secure or physical existence but were merely the passing virtual or intellectual rhetorical products of Modernity-in-argument, a faith that was always much much more interested in producing certainty/reducing uncertainty than in revealing the truth.
Consider what we mean when we claim to be "purifying" penicillium 'waste' water to obtain pure natural penicillin.
Penicillin molecules are hardly simple - still impossible for the best human chemists to synthesize economically or safely. While the waste molecule of water is a very simple molecule and easy to synthesize.
Or think of the long and uneven history of Nova Scotia's forests.
Initially only Nova Scotia's mast-making white pines were considered valuable and all the other trees mere junk. Then suddenly the best species for coal mine pit posts were the only trees considered valuable.
Until after WWII that is, when the mines all closed suddenly and now the former waste species of balsam firs because the only valuable tree when newsprint pulp mills became the sudden rage.
Uranium ores were once very expensively processed for tiny tiny amounts of radium, used for cancer treatments, and the rest chucked out as dross .
Until the A-Bomb (and the resulting hospital cobalt 'bomb') made the uranium valuable and the radium a mere costly impurity no longer competitive with newer ways to treat cancers with radiation.
Just as there never was any blood type and DNA evidence to support the rhetorical claims that the Aryans were particularly 'pure' and the Jews was particularly 'impure'.
Modernity worked that way : you simply presented the claim that your side was purer,simpler, brighter, more worthy than the other side and then drew your swords.
After the battle, if your side were the survivors, your claim was judged correct : might, not facts, was right...