Halakhah zu Dewarim 27:26

(3) And if (G–d forbid) this matter becomes hefker [inconsequential] to him, so that he does not take it upon himself to guard himself from it, he transgresses further a third arur (Devarim 27:26): "Cursed is he who does not fulfill the words of this Torah to do them," which is understood as his not accepting it upon himself to fulfill the entire Torah. And he is called a "mumar [heretic] in respect to one thing" because of this — gratuitously transgressing this grave issur, regarding this article of the Torah of the L–rd as hefker — being like any other "mumar in respect to the entire Torah." Therefore, his sin is too great to forgive. We have thus enumerated three arurin which are often attendant upon this evil trait.