Sforno on Deuteronomy 27:26/ספורנו על דברים כ״ז:כ״ו

1אשר לא יקים את דברי התורה הזאת לעשות אותם, who neither observes nor admits that they are all worth observing. but, who in his arrogance, considers some commandment or commandment of the Torah not worth his while observing. In the Talmud such a person is considered a “heretic because he rejects a single one of G’d’s commandments.”