Know! there are differences between Torah lessons. There is Torah which was not given even to be expounded. There is also Torah which was given to be expounded, but was not given to be written down. And there is [Torah] which was given to be written down. This is as the Sages, of blessed memory, taught (Gittin 60b) : Teachings communicated orally may not be transmitted in writing….

Anyone who knows how to distinguish and discern between the Torah lessons—which was given to be written down and which was not given to be written down—has the ability to discern the Jew among the gentiles. Even if a single Jew is standing among many gentiles, he is able to recognize him.

This mystery is hinted at in the verse (Hosea 8:12), “Many of My Torah teachings I have written for him; they have been regarded as something alien.” In other words, when one writes down “Many of My Torah teachings”—many more than is fitting, i.e., that which ought not be written down—then, “they have been regarded as something alien.” In other words, he is unable to recognize the Jew. He regards him as alien, because he appears to him to be a stranger and a gentile. And conversely, he might mistake the gentile for a Jew.

For that which mainly separates Jew from gentile is an aspect of that which was given not to be written down, the concept of the Oral Torah. As it is brought: This is why the Oral Torah was given. It was because [God] foresaw that the Jewish people would be in exile, and that the nations would reproduce the Written Torah for themselves. He therefore gave us the Oral Torah, which they cannot reproduce because it is oral.

It follows, that the main difference and advantage which the Jewish people has over the nations is in the aspect of the Oral Torah, which was not given to be written. And every single Jew has a portion in the Oral Torah, which was not meant to be written. Therefore, someone who can discern the Torah lessons given to be written down from those which were not given to be written down, is able to discern Jew from gentile—since this is their main difference.