Musar על סנהדרין 197:18

It is public knowledge to all of Israel, both young and old alike, that the Torah is read, with the sanctity of a sefer Torah, only when it is entirely written without even one letter missing. Only then is it called (Psalms 19:8): "The Torah of the L-rd is complete." But if a letter is lacking, it does not possess the sanctity of a sefer Torah, but only the sanctity of parshiyoth. So is it with the Jew. When is he included in the congregation of Israel, having a share in the world to come? Only when he believes that the entire Torah was given by the L-rd from heaven. But one who denies it — even one letter of it — is not in the congregation of Israel (see Sanhedrin 99a; Rambam, Hilchoth Teshuvah 3:5; Yoreh Deah 158).