The Basis to Accepting the Oral Law

Gemara: [We continue with the story of the non-Jew who wanted to convert but only believed in the Written Law and not the Oral Law and was rejected by Shammai. Hillel converted him and then took upon himself to teach the convert to accept the Oral Law.] The first day he taught him the letters aleph, bet, gimmel, dalet. The next day he switched the order. The convert asked: "That is not what you taught me yesterday!" Hillel said: "Isn’t it so that you trusted me? Trust me also regarding the Oral Law."

Ein Ayah: The main reason for those who try to destroy the holy covenant surrounding the Oral Law is that they are blind to the absolute dependence of the Written Law on the Oral Law. Since they do not realize its importance, they do not see the truth of its transmission to Bnei Yisrael. If someone realizes that it is impossible to not have an Oral Law, then he is not susceptible to the mistake of doubting it. It would be folly to leave behind the traditions passed on through our fathers to make up our own system of scholarship.
This is why Hillel impressed upon the convert the critical need for a reliable tradition from previous generations, which is accepted by the nation as a whole. If you do not accept the tradition regarding something as basic as the letters of the alphabet, no one will be able to read and approach the Written Law. With the need for such traditions, it must be that, in general, tradition is needed to know how to arrive at Torah rulings, and one must not question the foundation of Oral Law.
Since nature requires that there be a known tradition, whoever denies it, denies nature. There must be some sort of connection that links generations. Even those who initially denied the tradition found themselves with so many doubts until they were forced to eventually accept tradition. That is why we apply to them the pasuk, "They left the source of fresh water (i.e., the reliable tradition within the community of Bnei Yisrael) to quarry for themselves broken water pits (i.e., alternative traditions)" (see Yirmiya 2;13).
The comparison between the Oral Law and the alphabet is very appropriate. Whichever version of the names of the alphabet one would choose, one cannot learn how to read without being exposed to a tradition that teaches the rules. The originally presented tradition is the authentic one. Once a person understands this, he will understand that there is no real alternative. Hashem must have given the Written Law along with a tradition. A person will find within himself to accept it, as Hillel urged the convert to do.