The very first Pasuk of this week’s Parsha tells us of the Mitzvah of “Kedoshim Tihyu,” “You shall be sanctified.” Both Rav Sa’adya Gaon and Rashbatz, 2 of the earliest enumerators of the 613 Mitzvot in the Torah, both include Kedoshim Tihyu as a separate commandment. But this phrase is quite vague; what exactly does it mean to “be sanctified?”
The two most famous explanations are those of Rashi and Ramban. Rashi explains that Kedoshim Tihyu is a requirement to create a “fence around immorality.” This command has been upheld by Chazal in several forms (Yichud, Kol Ishah, etc.). Rambam seems to agree with this explanation, as he includes the laws of marriage and immorality in the section of the Mishneh Torah entitled “Kedushah”.
Ramban’s interpretation is that the Pasuk instructs us to avoid the common mistake of allowing something because it is technically permitted, even if it is against the spirit of the Torah. Gluttony, Nivul Peh, alcoholism, and other such actions are not technically prohibited, but are clearly not the Torah way. The Ramban explains that Kedoshim Tihyu teaches us to avoid such actions.
Ohr Hachaim comments that this Pasuk informs us that one who refrains from doing something wrong receives reward as if he did a Mitzvah (see Kiddushin 39b). It is not enough to do Mitzvot; one must also avoid doing Aveirot.
There is a misconception that might arise from this command. One might think that it is better to completely remove oneself from the world, as this would help a person become more sanctified (as some other religions believe). Both the Chatam Sofer and his son the Ketav Sofer oppose this idea. The Torah does not say that we should remove ourselves from the world (Perushim Tihyu); rather, we should sanctify ourselves (Kedoshim Tihyu) within this world by joining together to do Mitzvot and help each other. This idea is based on the statement of Chazal (Torat Kohanim 19:1, Vayikra Rabbah 24:5) that this entire section was sent to the complete assembly of Bnei Yisrael, including everyone. To achieve Kedushah, we must all join together.
Kli Yakar, elaborating on the ideas developd by Rashi and Ramban, adds that Kedoshim Tihyu means that one must sanctify himself with what is allowed by not overindulging in physical pleasures. While it is true that asceticism is not the Torah way either, there is an intermediate stage where the Torah commands us to become sanctified by avoiding certain things.
In order to be sanctified, one cannot simply be better than those around him. The Maggid of Dubno used to tell a story to illustrate this idea. A man was looking for a Talmid Chacham who would marry his daughter. He found one of the best scholars in the local yeshiva. Some time later, the father-in-law discovered that his son-in-law no longer studied as much as he used to. The son-in-law excused his behavior by claiming that he still learned more than anyone else in his area. This answer did not appease the father. Similarly, we must all strive to be the best we can, not the best relative to everyone else.
There are many more pages that could be written about Kedoshim Tihyu. Perhaps the reason there is so much to say is because the concept of Kedushah is so all-encompassing in Jewish thought. As Chazal say regarding this commandment, many Mitzvot in the Torah are dependent upon this one.