The Chofetz Chaim describes an apikorus as:
??????- “???????? ???? ????? ?????… ?????? ??? ???? ?? ????? ???? ?? ????? ??? ????? ???…”
As you know the vast majority of non-frum Jews reject the divine authorship of the entirety of the Torah. Many Torah statings they outright reject in accordance with their understanding of modern mores (i.e. the impressibility of toeiva and even the application of the death penalty for such activity, among many other rejected Torah portions.)
And the Chofetz Chaim tells us to mock apikorsum:
“???? ?????? ?????? ??? ??? ????????? ????? ?????? ??????? ??? ?????? ???? ??? ??????”.
Thus, this would apply to most non-frum Jews, specifically those that reject even one pasuk in the Torah as coming directly from G-d Himself.
And as Rav Chaim Brisker famously said, ah nebech apikorus is oich an apikorus; even if they don’t realize rejecting one word of Torah makes them an apikorus, it still makes them exactly that.