More from Rambam, Mishneh Torah 3:8.
‘There are three individuals who are considered as one “who denies the Torah”:
a) one who says Torah, even one verse or one word, is not from God. If he says: “Moses made these statements independently,” he is denying the Torah.
b) one who denies the Torah’s interpretation, the oral law, or disputes [the authority of] its spokesmen as did Tzadok and Beitus.
c) one who says that though the Torah came from God, the Creator has replaced one mitzvah with another one and nullified the original Torah, like the Arabs [and the Christians].’
There are actually passages in Chazal and some other rishonim that violate the strict meaning of (a). One would have to make a big stretch to interpret (b) as referring to someone who question’s a non-halachic teaching of Chazal as Rambam himself introduced non-literal interpretations that don’t coincide with those of Chazal, and his son Rabbi Avraham explicitly permits non-literal interpretations. (c) is clearly directed towards Muslims and Christians.