The most powerful day
כי-ביום הזה יכפר עליכם לטהר אתכם מכל חטאתיכם לפני יקוק תטהרו
For on this day [of Yom Kippur] it shall be atoned for you, to purify you, from all of your sins; purify yourselves before Hashem!
Yom Kippur is one of the most intense days of the year. We spend the entire day involved in prayers and supplications. We fast, and refrain from physical pleasures. We (hopefully) perform teshuvah, repentance with sincerity and a broken heart. With this, we hope to repair the damage we inflicted to our relationship with our Creator. After all of this, a person may wonder: How can I know that my repentance was accepted?
Rabbi Dessler records a test he was taught for a way to gauge this. Chazal teach us that our sins cause a spiritual timtum halev, blockage of the heart. This is a sort of contamination which distances us from being able to receive and tap into holiness. What this means then is if a person’s teshuvah was successful, their sin was completely cleansed. Therefore, any spiritual contamination would be removed as well. If a person can sense that their blockage is gone, they can be assured that their teshuvah was accepted.
However, this test is not so simple. It seems to be assuming that teshuvah has the power to retroactively uproot the sin from its source, taking the contamination with it. Indeed, this is how some understand the power of teshuvah. However, others understand it differently. They say that teshuvah works from that moment going forward, but doesn’t affect the past. A person is forgiven for their misdeeds, but they aren’t erased. Seemingly then, the spiritual contamination would still be there. That means if after Yom Kippur a person feels like they did before they did teshuvah, that wouldn’t indicate anything.
However, it could be this test works according to all opinions. Maybe teshuvah on Yom Kippur is different than any other day of the year. Teshuvah on a regular day would result in a forgiveness that was from that moment going forward, whereas teshuvah on Yom Kippur according to all opinions would work retroactively. The day itself is a day of repentance, that coupled with the person’s personal teshuvah will remove the sin completely, as if it never existed . As such, any spiritual contamination would be cleansed as well. Rav Dessler’s test should work then according to everyone. Thus is the power of Yom Kippur, as well as teshuvah.
Gemar chasimah tovah
 Based on L’Dofkei BaTeshuvah on Hilchos Teshuvah 1:3:94 and Biurim ad. loc. s.v. אין מזכירין
 Leviticus 16:30
 Michtav MeEliyahu I p. 266
 He records this in the name of Rav Yitzchok Blazer, one of the primary disciples of Rav Yisroel Salanter
 Yoma 39a
 Chibbur HaTeshuvah L’Meiri 1:12; Beis Elokim Sha’ar HaTeshuvah Chapter 1; Mesillas Yesharim Chapter 4; Resisei Laila § 53
 Rabbeinu Yonah to Avos 3:16; Peirush HaGra to Song of Songs 1:16
 See Teshuvos Beis HaLevi II Derush 15, who says similarly
 Rav Chaim Kanievsky shlita, in his marginal notes to L’Dofkei BaTeshuvah, writes: נכון
 I’m surprised Rav Dessler thought to record this test for today’s generation. I feel many people today are so far removed from sensing spirituality and kedusha that they can’t detect their spiritual contamination, let alone it’s (hopeful) departure after Yom Kippur