Doubtful Belief

The Gemara in Shabbos (31a) tells the story of how a Gentile wished to convert but stated that he only believed in the Written Torah but not the Oral Torah. Shammai sent him away but Hillel converted him and then convinced him about the Oral Torah. Rashi (sv. geireih) explains that the convert did not reject the Oral Torah but just did not believe in it (שלא היה כופר בתורה שבעל פה אלא שלא היה מאמין שהיא מפי הגבורה). What does that mean? Either he did or did not believe in it.

R. Yosef Engel, in his Gilyonei Ha-Shas ad loc., quotes the Chiddushei Gur Aryeh who suggests based on this Rashi that one who rejects the Oral Torah is not considered a heretic. Otherwise, how could Hillel have converted a heretic? R. Engel disagrees and explains that one can only be a heretic if he knows the Torah and then rejects it. This Gentile had never learned the Torah properly and therefore could not be considered a heretic for rejecting it. See the Rashash's glosses for a similar approach.

R. Norman Lamm, in his classic essay "Faith and Doubt" (recently reprinted in a new edition of Faith & Doubt, pp. 16-17), has an explanation that does not go as far as the above. He suggests that this Gentile had doubts but not full-fledged disbelief. Someone who believes to the contrary of a fundamental belief is a heretic but someone who merely has doubts does not reach the status of a heretic.