Rashi explains this refers to rehgc rpuf, one who will eventually deny the basic tenets of religion or the existence of Hashem. This commentary does not seem consistent with the sequence of the text. After the Torah enumerates various punishments to befall those who sin, it states: “And if after all this you will [still] not listen to Me and walk contrary to Me (hrec)” (26:27). Rabbeinu Yonah characterizes hre as one who dismisses the punishments as “accidental” happenings, which are not relevant to him at all. According to the text, this sin is even more egregious than the previous ones. This seems enigmatic! Is there a more reprehensible sin that denying the existence of the Creator? One who denies Hashem’s existence will surely dismiss His punitive measures as merely random occurrences!
Horav Yechezkel Levinstein, z.l., suggests that the concept of rehgc rpuf applies to anyone who deviates from total dedication to the Divine. A Jew must place his complete trust in Hashem and not in any other ideology. This was indicated by the sin of the Golden Calf. Bnei Yisrael were seeking an intermediary to replace Moshe. The concept of an intermediary is blasphemous. Hashem is a personal G-d, Who responds directly to each individual without any displacement.
Anyone who does not have absolute total faith in Hashem is tainted by the stain of rehgc rpuf/ Thus, one who dismisses Hashem’s punishments is essentially reinforcing his blasphemous beliefs.
