"And Hashem spoke to Moshe at Mount Sinai saying: ... When you enter the land which I am giving you, and the land shall rest a Sabbath for Hashem" (VaYikra 25:1-2).
Since at this point the mishkan already was standing and Hashem was communicating with Moshe Rabbeinu from the mishkan itself (VaYikra 1:1), no longer on Har Sinai - combined with the fact that none of the other multiple commandments from the beginning of Chumash VaYikra until this point were stated as being at Har Sinai - the commentaries ask why Sinai is mentioned at this point. (See Rashi, Ibn Ezra, Seforno, and Ramban among others.)
Ibn Ezra, utilizing the principle of ein mukdam umeuchar baTorah - the Torah does not always follow a chronological order, offers a unique, complex explanation as to why Har Sinai is mentioned here. He writes that the commandment of shemitta was actually given (both from Hashem to Moshe and from Moshe to Bnei Yisrael) much earlier at Har Sinai before the mishkan was built. Why then was it placed here? He explains that the Torah was given to the Jewish people as a bris, a covenant, as described in parshas Mishpatim at which time Moshe read the Seifer HaBris, the Book of the Covenant, to Am Yisrael, an altar was built and sacrifices were offered. In addition, the blood of the korbanos, referred to as "dam habris", was sprinkled on the people by Moshe Rabbeinu (Shemos 24:6-8). This covenant actually included the blessings and curses of parshas Bechukosai which are frequently called a bris (VaYikra 26:9,15,25,44,45. Also see Ibn Ezra to 26:25.) However, the Torah delayed their recording to this point instead of placing them earlier on when the bris of Har Sinai was described. Once again, why the delay? Ibn Ezra answers that concerning the prohibition of arayos, forbidden relations, listed in parshiyos Acharei-Mos and Kedoshim, given after Har Sinai, the Torah states that this is "mitena"ai ha"aretz", among the stipulations necessary to keep in order to stay in Eretz Yisrael; if the Jewish people violate arayos, the land will "vomit them out" (18:28, 20:22). Shemitta is also mentioned in the tocheicha as being "mitena"ai ha"aretz", violation of which can lead to exile (26:34:43, Avos 5:9). In order to juxtapose the two sets of commandments whose observance allows for continued stay in the land, and whose violation leads to the opposite, chas v"shalom, the Torah records the elaboration of the laws of shemitta after the commandments concerning arayos and not next to the description of the events of Har Sinai where they were actually commanded.
Ramban accepts Ibn Ezra"s premise that there is a linkage between observance of the laws against forbidden arayos and the mitzvah of shemitta as conditions to hold onto Eretz Yisrael, but disputes his application of ein mukdam umeuchar baTorah. Ramban boldly asserts that the original bris of Sinai, as a result of the sin of the Golden Calf was seemingly nullified. (See below that he strongly implies that it was not actually eliminated but was treated by Hashem, to some extent, as if it were so.) As a result, a new treaty was forged between Hashem Yisborach and the Jewish people. It was this new bris which forms the content of the alos and kelalos of parshas Bechukosai. Since the original covenant had been, to some extent, violated, Hashem insisted on strengthening the force of the bris with threats of exile and other suffering if its content were to be violated by Bnei Yisrael again. This concept of a new bris is directly referenced after the cheit ha"eigel in parshas Ki Sisa: "Behold I am forging a bris; before your nation I shall perform wonders..." (Shemos 34:10). Its content was revealed to Moshe at Sinai and was related to the Jewish people in our parshiyos of Behar and Bechukosai. Shemitta had been one of the original mitzvos that was a prominent component of the bris as recorded in parshas Mishpatim (23:10-11). It became an even more prominent part of the new covenant with more detailed elaboration of its laws. The mention of "Behar Sinai" with regard to shemitta in our parsha, according to Ramban, refers to the commandment that G-d told Moshe in conjunction with the giving of the second luchos after the cheit ha"eigel as part of the process of the renewal of the covenant. It was only taught to the Jewish people at this point.
What emerges then from the pesukim as elaborated upon by these commentaries is that the Nosein haTorah did not just command us in a set of mitzvos but insisted that we enter into a mutual treaty with Him. The famous statement of "na"aseh v"nishma" (Shemos 24:7) represented the Jewish people"s acceptance of the original covenant of Sinai. The phenomenon of a "national convention" leading to a covenant with G-d repeats itself throughout Jewish history. Earlier, the founding father of the Jewish nation, Avraham Avinu, entered into two brisos with Hashem, the Bris bein HaBesarim (Bereishis 15) and the bris mila (ibid. 17). In addition to the two Sinaitic ones mentioned by Ramban, a third covenant was established with the Jewish people at arvos Moav (Devarim 28:69, 29:9 ff.). (See the commentaries including Ramban and Abarbanel (ad loc.) as to the need for an additional bris at that point.) In the days of Mordechai and Esther, our Sages teach us that the Jews then reaffirmed their commitment to the bris. On the verse "kiyemu vekibelu hayehudim" (Esther 9:27), Rava states (Shabbos 88a): "they reaccepted [the Torah] in the days of Achashveirosh; they reaffirmed that which they had already accepted." Mori v"Rabi Rav Hershel Schachter shlita quoted a Geonic version of the Gemara (Pesachim 68b) that the meal on Purim celebrates this renewed kabbalas haTorah. In the days of Ezra and Nechemia, the Jewish people, after separating from their gentile wives and non-Jewish children and committing to full observance of Shabbos, committed to and even signed the "bris amana - the covenant of loyalty" (Nechemia 9,10). So too in the period of the final redemption, in the era of Messianic perfection, Hashem promises to form a new covenant, a "bris chadasha", with the Jewish people, reaffirming for all eternity the covenant of Sinai which will never again be abrogated (Yirmiyahu 31:30-33).
Ezra HaSofer ordained that the two tochaichos of Bechukosai and Ki Savo be read before Shavuos and Rosh HaShana, respectively (Megilla 31b). On a simple plane, they warn us of the dire consequences of abandoning the source of life itself, the Torah and its mitzvos, before the day of the giving of the Torah and before the day of Judgment, respectively. But this takkana also highlights the historical entry into a bris with HaKadosh Baruch Hu and the need to renew our commitment to that covenant. Rav Y. D. Solovetichik zt"l (Yemei Zikaron) highlighted the emphasis on bris in the musaf prayer of Rosh HaShana. Almost all the pesukim quoted in the blessing of zichronos refer to bris, and the blessing ends with the phrase "zocher habris". Shavuos, the day of both Matan Torah and Kabbalas HaTorah stresses this same theme.
The concept of tochaicha, intertwined with the bris of Sinai, brings to mind frightening images of Divine wrath and punishment, exile and suffering, unmentionable tragedy and pain. The custom is to read this portion in a low voice indicating our fervent wish not to have to undergo its prophecy of doom. But the giving of the Torah on Shavuos is midrashically compared to a marriage between HaKadosh Baruch Hu, the chassan, and Knesses Yisrael, the kalla (see Ta"anis 4:8, Rashi to Shemos 34:1). Many of the customs of a Jewish wedding are derived from Har Sinai: the chuppa parallel to the mountain over us, marching down to the chuppa with fire reminiscent of the lightning of Sinai, and the giving of a ring parallel to the giving of the luchos. Maharsha (Kesuvos 7b) even writes that the ending of the blessing of eirusin, "m"kadeish amo Yisrael al yedei chuppa vekiddushin" can be translated as "Hashem married the Jewish people through chuppa and kiddushin" (the luchos and Har Sinai)! How can the imagery of the happiness of a marriage be reconciled with the fright of tochaicha.
A fundamental insight I heard from Rav Michael Rosensweig shlita[1] can perhaps resolve this seeming contradiction. Non-Jewish marriage (from a Jewish law perspective) is a "common-law" marriage. A couple decides to live together as husband and wife. Divorce is accomplished by either spouse initiating a separation. (See Rambam, Ishus 1:2, Melachim 9:8.) Jewish marriage, by contrast, is based upon kinyan: "Ha"Isha nikneis bishlosha d"rachim - a woman enters into the marriage kinyan via one of three methods" (Kiddushin 1:1). In the language of Rambam (Ishus ibid.) "once the Torah was given, Israel was commanded that if a man wishes to marry a woman, he should first be koneh her in the presence of witnesses and only then will she be his wife..." This concept of kinyan in marriage has often been misunderstood. Any seasoned student of the Talmud immediately recognizes that a woman is not "acquired" by her husband as property. What then is the "kinyan" of marriage?[2] A Jewish marriage is characterized by the commitment that marriage entails. Unlike non-Jewish marriage which is formed without kinyan, or without commitment, which can be broken at will, Jewish marriage requires this commitment and is not so easily broken. The fact that the punishment for adultery is so severe is indicative of how intense the bond between husband and wife really is. This, in turn, is reflective on a microcosmic level of the macrocosmic "marriage" between HaKadosh Baruch Hu and the Jewish people. It is premised upon mutual commitment and not flippant, ephemeral, spur of the moment will or desire alone. The disastrous consequences predicted by the Torah for abandoning its precepts are reflective of how serious and everlasting this fundamentally loving relationship is. Ramban earlier carefully states that even after the cheit ha"Eigel "ke"ilu nisbatela habris - it was as if the covenant had been nullified"; in reality, it was never nullified as it is eternal. This is why, explains Ramban, no korbanos or sprinking of the dam bris on the nation was necessary for the second bris since it was merely a reaffirmation of the first. I heard from Rav Schachter shlita in the name of Rav Nachman miBreslov that one meaning of the breaking of a glass under the chuppa is to represent the breaking of the luchos rishonos; even after they were broken, the covenant continued with the luchos sheniyos!
The urgency of Torah and mitzvos as reflected by the tochaicha is indicative of the intense, eternal connection of the Creator of the World and his beloved nation. The eternal bris and its fundamental nature is expressed so poignantly by Yirmiyahu (33:25) "im lo brisi yomam valayla, chukos shamayim va"aretz lo samti - if not for my eternal covenant, I would not have formed the laws of heaven and earth!" Chazal explain this bris to be alternatively Torah (Pesachim 68b) or Avoda (Megilla 31b). The upcoming holiday of Shavuos highlights the first. The celebration of Yom Yerushalayim before Shavuos commemorates, in a sense, the second meaning of bris. Not only do we celebrate the great miraculous salvation Hashem orchestrated for his people during and after the Six-Day War but the reconquest of the ancient Yerushalayim and the Har HaBayis which, G-d willing, will be the first step in the eventual road to the re-establishment of the Beis HaMikdash and the service of Hashem there bimheira beyameinu!
[1] Introductory shiurim to Kiddushin and Kesuvos on yutorah.org. The presentation is as I understood his words. Any error in formulation is my own.
[2] Much has been written as to its precise nature in Rishonim and Acharonim. Here, we focus on its hashkafic implications as I understood from Rav Rosensweig"s shlita shiurim.