In the famous story [Shabbos 31] of the ger who came before Hillel in order to convert and he said that he only believes in Torah Shebichtav but not Torah Shebial Peh, Rashi explains that he believed in Torah Shebial Peh but just didn't believe that it was from Hashem. What compelled Rashi to say this? Why don't we just understand that he didn't accept Torah Shebial Peh at all??
Explains Rav Hutner: