He who eats choice meats and drinks fragrant wines in order to expand his mind and better serve Hashem and study His Torah or to perform the mitzvah of enjoying Shabbos and yom tov is able to refine the essence of the meat and wine and release it from the husk known as “klippas noga.” This is as pleasing to Hashem as a fragrant burn offering… However, he who is a glutton and gorges on meat and wine to satisfy the desire of his animal soul alone...causes the essence of the meat and wine within him to descend into the completely evil klippos. Then his body acts as a vehicle for the klippos — at least, until he repents through teshuvah and serving Hashem and studying His Torah. Since the meat and wine were kosher, they can be elevated together with him when he returns to Hashem.

This is the meaning of “heter,” which connotes permitted foods. Literally it means “unfettered” or “unbound” because they are not bound to the exterior negative forces, as opposed to nonkosher foods, which are “assur,” forbidden — literally, “bound” to the negative forces — and cannot be released to ascend to Hashem... Any forbidden foods that a person might have eaten already will remain bound to the negative forces until the day when Hashem ends all death or until the person repents and does such a lofty form of teshuvah that his willful transgressions are transformed into actual merits.

This form of teshuvah is a teshuvah done out of an intense, fierce love that comes from the depths of his heart and out of such a passion to attach himself to Hashem that he is like a parched soul in a wasteland. He feels as if until today he dwelled in a desert wilderness far from Hashem where he thirsted for Hashem’s presence.

This is why our Sages taught that in the place where a ba’al teshuvah stands, not even great Tzaddikim can stand (Berachos 34b), and such a great and lofty teshuvah as this can transform willful transgressions into merits and release the bound essence in forbidden acts and foods (Yoma 86b). Teshuvah that is not done out of love, however, though it is sincere and surely Hashem will forgive him, is unable to transform transgressions into merits.

(Tanya, Sefer shel Beinonim)

The Torah’s definition of teshuvah is simply disengagement from the act of sin — to stop doing the sin. This is the definition that the Gemara cites in chapter 3 of Sanhedrin and in Choshen Mishpat (end of siman 34) regarding the testimony given by a witness who is a sinner. (A sinner’s testimony is invalid, so the Shulchan Aruch asks, if the sinner who was disqualified from giving testimony now wishes to repent, at what point is his teshuvah accepted and his testimony valid? The answer is that he must stop doing the sin.) This means that he must decide in his heart, fully and sincerely, that he will never again do those things and transgress the King’s commandments, whether positive or negative commandments. This is the primary definition of teshuvah — not fasting, as most people believe.

(Tanya, Iggeres HaTeshuvah)

Someone who gets sick from repeatedly fasting, especially nowadays, should refrain from doing so for the sake of teshuvah (this does not refer, of course, to halachically mandated fasts but to one who takes upon himself to fast as part of his repentance). He should refrain from fasting even if he has committed severe transgressions such as those that incur capital punishment or kares (spiritual excision), and even if he has transgressed many positive commandments and prohibitions. All the more so if he is learned and can study Torah — if he fasts, he is considered a sinner and will be punished for fasting because it weakens him and prevents him from properly learning Torah.

(Tanya, Iggeres HaTeshuvah)

When we do teshuvah, Hashem, in His great compassion, forgives us immediately. This fact is clearly demonstrated by the blessing of forgiveness that we recite when we pray Shemoneh Esrei. First we ask for Hashem’s forgiveness for our sins, and immediately after we pronounce, “Baruch atah Hashem chanun hamarbeh lislo’ach — Blessed are You, Hashem, the Compassionate One, who forgives abundantly.” In Jewish law, the general rule of thumb is that when in doubt we do not recite a blessing. We can conclude that the very fact that we recite this blessing clearly indicates that there is no doubt that Hashem pardons us after we have beseeched His forgiveness.

(Tanya, Iggeres HaTeshuvah)

Rav Shneur Zalman of Liadi, the founder of Chabad Chassidus, taught that teshuvah is not just for sinners. Everyone must do teshuvah, because the very definition of teshuvah is “return.”

Teshuvah is the process whereby we return the soul to its root source, where she was once bound up in her Creator and attached to the Source of life, to Hashem. She descended from that lofty plane and enrobed herself in this physical world. Any spiritual achievements that she attains while in this world pale in comparison to her degree of spirituality when she was on High because in the physical world she is bound by such constraints as time and place.

The only way to return her to her Source is by “crying and calling out to Hashem from our distress and constraints” (Tehillim 107:19). What are these constraints? Being bound by time and space; they are in and of themselves boundaries to achieving lofty spiritual levels. Out of the very bitterness that we feel about these barriers, our love and desire for Hashem will grow correspondingly and exponentially. Thus “from our distress” we will yearn to attach ourselves to the light of the Infinite One. This is the “light that draws benefit from the darkness” (Koheles 2:13).

This is what our Sages meant when they said, “In the place where the ba’al teshuvah stands, even the righteous cannot stand” (Berachos 34b). The Zohar explains that ba’alei teshuvah (those who return their souls to their root source) have greater strength than the Tzaddik. This is because the light of the Infinite One is truly revealed in their souls to the point where their animal soul, their body’s vitality, and their abilities and traits are transformed and returned to Hashem, as darkness is transformed into light.

(Torah Ohr, Vayechi)

Some people make the mistake of thinking that teshuvah is meant only for sinners. They mistakenly believe that only lowly, unworthy people who have transgressed the commandments must repent. The truth is that teshuvah is a much loftier process than just rectifying sins.

The Zohar teaches us (Naso 122a) that the word teshuvah is a compound of the two words “tashuv hei,” which means, “Return the letter hei to its proper place.” There are two types of heis (found in Hashem’s four-letter Name, Yud-Kei-Vav-Kei). The first is called the “hei ila,” or “upper hei (and it corresponds to the sefirah, attribute, of binah, insight). The second is the “hei tata,” or “lower hei” (which corresponds to the lower sefirah of malchus, kingship). When a person interrupts his Torah study or prayers with mundane thoughts or speech, they separate the heis from the divine Name, so to speak. The Tzaddikim must then repair the spiritual damage and return the heis to their proper place.

Thus we can say that those who study Torah and dwell in a house of study have the power to restore the letter heis to their proper place. Generally speaking, the study of the Written Torah corresponds to the hei ila, and the Oral Torah or Talmud corresponds to the hei tata. Specifically, thoughts of the mind, such as meditating on the greatness of Hashem and having intense concentration during prayers, corresponds to the hei ila. Through these meditations one can restore that hei. Speech, such as pronouncing the words of the Torah that one is studying and praying, corresponds to the hei tata. Teshuvah, then, is about returning the heis to their proper place through these meditations and words of Torah.

(Ma’amarei Admor HaZakein HaKetzarim)

A primary form of teshuvah is to verbally confess the sin you have transgressed, describing it in detail. Why do we do this? When you sin, you create an adversary — a prosecuting angel — which is called by the very same name as the sin you committed. For example, if someone eats pork, the prosecutor is named “You Ate Pork.”

The prosecuting angel draws its life force from the very letters that spell his name — that spell the sin that was committed. This is how the first man, Adam, named the animals: when he looked at them, he discerned the letters that gave them life. For example, when he looked at a horse, he discerned that the letters samech-vav-samech give it life, so he named the horse “sus” (סוס).

Therefore when a person confesses and regrets his actions saying, “I no longer wish to do this sin anymore,” he is removing the life force from the prosecuting angel that he created with that sin, effectively killing it, because he has taken away the letters that form his name that give him life.

(Ma’amarei Admor HaZakein HaKetzarim)

To understand why the power of teshuvah is so great, we must understand that teshuvah is a transformation and its power lies in a great novelty. A precious stone, for example, is valuable because it reflects light and this makes it beautiful. Now you might ask, “Doesn’t a candle shine more brightly than any precious gemstone?” But the true value of a gemstone is the fact that even though it is only a rock, it still shines! This is the great novelty — that a piece of rock can shine — and that is why it is so precious.

Similarly, a person’s hardened heart is called a “heart of stone.” When he transforms his will and changes himself to serve Hashem, his heart shines. This is a great novelty and very precious in Hashem’s eyes. When our hearts of stone shine, we call them precious gemstones and we make them into “jewelry” to adorn the Shechinah who is like a bride. There are many kinds of jewelry and precious gems, some more valuable than others. When a bride is bejeweled with all these kinds of precious gems, then she is pleasing and beautiful to her husband (i.e., HaKadosh Baruch Hu, the male aspect of Divinity).

(Ma’amarei Admor HaZakein HaKetzarim)