By way of introduction to Iggeret HaTeshuvah it should be noted that the Alter Rebbe is known as “Master of the Tanya and Shulchan Aruch.” The Rebbe once remarked that “Master of the Tanya” means that the Alter Rebbe is an arbiter in the esoteric dimension of the Torah, and “Master of the Shulchan Aruch” signifies that his halachic rulings are authoritative.
Furthermore, the previous Lubavitcher Rebbe once stated in a public address that the four parts of Tanya correspond to the four sections of the Shulchan Aruch. In this connection the Rebbe gives an explanation — both according to Chassidut and according to the revealed strata of the Torah — of the relation between the third part of Tanya, Iggeret HaTeshuvah, and the third section of the Shulchan Aruch, Even HaEzer.
According to Chassidut the relation between the two is clarified by a statement in ch. 4 of Iggeret HaTeshuvah — that the lower and higher levels of teshuvah (which together encompass all the degrees of repentance) are respectively indicated by the lower and higher letters hei of the ineffable Name of G‑d. In terms of their spiritual personality, so to speak, these two letters are feminine: both are receptors, the higher hei (representing the level of Binah) being impregnated by Chochmah, and the lower hei (representing Malchut) being impregnated by the six emotive Sefirot. This feminine element connects Iggeret HaTeshuvah with Even HaEzer, which codifies the laws involving women.
As to the revealed plane of the Torah, we find that the Talmudic Tractate Gittin, which deals with the laws of divorce, precedes Tractate Kiddushin, which deals with the laws of marriage. In the introduction to his commentary on the Mishnah, Maimonides explains this order by quoting the verse, “When she leaves his house she may go and marry another man”; here, too, divorce precedes marriage. Historically, as well, the Midrash points out that the root of the word used by the Torah to say that G‑d banished Adam from the Garden of Eden ( ויגרש ) is the same as the root of the word for divorce ( גרושין). Accordingly, the Sages compare his state to that of “a Jewish divorcee,” who is permitted to remarry her former husband. And indeed, when G‑d later gave the Torah to the Jewish people he “sanctified us ( קדשנו) with His commands.” In the Holy Tongue this verb shares a common root with the word for marriage, or betrothal ( קדושין). In this connection the Alter Rebbe said above that G‑d’s having “sanctified us with His commands” parallels what a man declares when betrothing a wife: “You are hereby consecrated unto me.”
This dynamic — marriage in the wake of divorce — is echoed in the spiritual use of these terms. The connection to teshuvah is thus readily apparent: A “marriage” is conceivable after a state of “divorce” only when there was teshuvah in the interim. For as the Alter Rebbe stated earlier on, “Indeed, it is impossible for the wicked to begin to serve G‑d without first repenting for their past.”
In Scripture, too, we find repentance depicted as the reconcilement of a divorced couple, culminating in remarriage. For sin banishes the Shechinah, the Divine Presence, the Mother of all souls. In the words of the prophet, ובפשעיכם שלחה אמכם — “Because of your transgressions was your Mother sent away.” This is the selfsame verb that the Torah uses for divorce: ושלחה מביתו — “And he will send her away from his house.” And it is repentance that undoes this spiritual divorce, to the point that G‑d can ask His people the rhetorical question: “Where is your mother’s bill of divorce?” — for as a result of His people’s repentance, the divorce is annulled.
In the plainly manifest levels of the Torah as well, there is explicit evidence in the Gemara that repentance resembles remarriage following divorce. R. Yochanan teaches that repentance overrides a prohibition stated in the Torah, and cites the following verse: “If a man sends away his wife and she leaves him for another man, will he return to her again?.... Yet though you have strayed..., return to Me!” Thus, argues R. Yochanan, G‑d is saying here that repentance overrides the prohibition that “her first husband...may not remarry her” [if she married another man in the interim]. Here too, then, remarriage following divorce is a paradigm of repentance.
Thus, there is a clear correspondence between the third part of Tanya, Iggeret HaTeshuvah, and the third section of the Shulchan Aruch, Even HaEzer, which deals with the laws involving women.