I
"Cursed is the one who will not uphold the words of this Torah to perform them" (Devarim 27:26). They accepted the entire Torah upon themselves with a curse and with an oath (Rashi, ibid).
Ramban interprets this acceptance as follows: he acknowledges the mitzvos in his heart and they are true in his eyes. If he denies one of them, or if he thinks that even one of them is annulled ["b'taila le'olam"] at any future time, he is cursed. If, however, he sins because of lust, or omits a mitzva because of laziness, he is not included in this curse. This is a curse [cherem] against rebels and deniers of the Torah and its eternity.
Ramban then quotes the Yerushalmi (Sotah 7:4) which explains that the one who "will not uphold" is one who learned, taught, observed and performed the Torah, but he was able to strengthen the observance of Torah [among others, hareshaim hamevaltin osah] and did not strengthen it, and he is included in the curse. Ramban concludes: This is close to our [first] explanation.
How are these two explanations (denying the eternity of Torah and one who did not capitalize on an opportunity to strengthen its observance) related? The Artscroll edition of the Ramban (fn. 43) connects them as follows: upholding does not refer to personal performance, but rather to affirming the obligatory nature of the Torah, either in belief (Ramban) or by enforcing it (Yerushalmi).
Perhaps there is a deeper connection between the two explanations. The Ramban excludes a lustful sinner from the curse, because such a sinner believes in the Torah and its eternity but does not control his desires. However, a person who does not sin, does control his desires (or has none), and performs mitzvos as a traditional Jew, but does not believe in the divinity or the eternity of the mitzvos, would presumably be considered by the Ramban to be a denier of the Torah and its eternity. Such a person, therefore, would be included in the curse.
The Yerushalmi goes a step further than the Ramban, as follows: one who believes passionately in the Torah wishes fervently that it be observed universally. If he has the ability to increase the number of Jews who observe the mitzvos but does not do so, his belief may lack certainty or passion. This is close to the Ramban's explanation, because both understand the curse to relate to one who lacks belief rather than one who lacks performance.
II
The Yerushalmi states that King Yoshiyahu ripped his garment [when the sefer Torah was discovered, Melachim II 22:11] and said "Alai l'hakim - It is upon me to uphold". Ramban explains that the king and the princes are able to uphold the Torah and to enforce its observance by those who presently ignore it [b'yad Hamevatlin Osah]. As such, if they do not do so, they are cursed. Yoshiyahu recognized this and ordered the destruction of all the idols throughout his kingdom (23:3-15), and an unprecedented observance of korban Pesach (23:21-25).
Enforcement of this type is, of course, impossible nowadays. Nonetheless, even as we lack power to forcibly spread Torah observance, we do not lack influence. This is the mandate to engage in kiruv, to bring the non-observant closer to Torah performance and belief.
Recently questions have been raised, even among some traditional Jews who observe mitzvos, about the eternity of the Torah. Others have questioned the morality or eternity of a Torah law which modern society regards as immoral or outdated. Some actually reject the concept of a divine, immutable Torah. These attitudes may be included in Ramban's understanding of "aror asher lo yakim", even if no laws that require or forbid specific actions are violated.
Here, too, we are powerless to enforce, but we are able, and therefore responsible, to influence others. We must unapologetically affirm our belief in the divinity, morality and immutability of Torah. If we cannot convince the deniers or doubters, we must at least strengthen the believers. As the Yerushalmi states, anyone who is able to strengthen the Torah and does not do so is one who "does not uphold" it. May each of us, like Yoshiyahu, recognize our responsibility and exclaim "Alai hakim - it is upon me to uphold."