(Excerpted from the article, "From Misnagid to Rebbe" by Rabbi Naftali Flintenstein, as it appeared in the Elul 5770 edition of Kolmus magazine. It tells the story of Rav Tzaddok HaKohein of Lublin.)
One of the focal points of Rav Tzaddok's writings is the topic of sin and teshuvah. As a talmid of the Izhbitzer Rebbe, he pursued each topic with the goal of uncovering its inner truth, seeking to uncover why we sin and how we can recover once we have fallen.
Rav Tsaddok builds his own principles on this idea, writing, "The sins of Klal Yisrael are not an inherent failure. Rather they are like a superficial stain falling on a cloth, as it says, 'And your sin is a stain...' If we rub and wash, the cloth will be clean [once again]" (Likutei Maamarim pg. 84).
...He continues: "If a person has great physical desire, he should not become depressed, thinking he is flawed for having such desires. Rather, he should realize that [the desire] represents the characteristics necessary to love Hashem greatly and to eagerly seek truth... Thus it is written that in Days to Come, [Hashem will remove the yetzer hara, and ] tzaddikim will see it as a great mountain and reshaim will see it as a flimsy hair. Both are true, for the desires of tzaddikim are extremely strong." (Tzidkas HaTzaddik, p.44)
"Every nisayon that a person faces is new---it contains challenges that have never been faced before... Likewise, the nisyonos that face each generation are new challenges that man has never faced before, and it therefore takes extreme efforts to overcome them." (Yisrael Kedoshim, p.5)
"When a person sins, he should know that it can prepare him to be successful in the same matter. If he merits, he can ascend to great heights, and the greater the sin the yetzer hara seduced him into transgressing, the higher the level he can subsequently attain." (Tzidkas HaTzaddik, p.76)
"Each person must know that the area in which his yetzer hara is most potent is the area in which he has the greatest potential for purity and righteousness. The areas in which he faltered the most are the exact areas in which he can become clean and pure-hearted... This is the tikkun of his soul, for each person was created to repair some specific thing that only he can fix... Thus it says that no man can follow the Torah fully unless he first stumbles in it, for when he stumbles he realizes that the area in which he stumbled is the area in which he must make a tikkun." (Tzidkas HaTzaddik, p. 49)