Why do theist often drop the letter s when referring to atheists?

[Neo-Scholastic]

In Western Europe during the Middle Ages, almost no one read alone silently (doing so could be legacy of monastic life is one theory) Otherwise, most vernacular reading was performative, i.e. read out loud to an audience. Spelling varied depending on the dominant accent. That is presumably why English has so many weird spelling rules and exceptions. A person wasn't truly and duly educated unless they could spell a word three different ways.