We discussed the purity that comes from studying Torah and keeping Shabbos. Another primary source for taharah in our generation is mikvah. We don't have a parah adumah today, but we still have mikvah, a source of immense purity for the Jewish nation. The Baal Shem Tov zt’l said that he achieved his levels through mikvah.
Mikvah purifies from tumah and from aveiros, which are a type of tumah. The Or Zarua discusses a baal aveirah. He writes, "He needs tevilah to cleanse himself from the aveirah, because the aveirah made him tamei. Because aveiros make people tamei. As the Midrash (Tehillim 51:4) states, הרב כבסני מעוני ומחטאתי טהרני, 'Cleanse me a lot from my sin. Purify me from my iniquity.' This teaches us that committing an aveirah is like becoming tamei to a dead person. As I learned from Rabbeinu Simchah, 'All people who want to do teshuvah must go to mikvah.'"
The Radvaz (vol.3 415) states, "To do teshuvah for every type of sin, one needs mikvah first." This is one of the reasons everybody goes to mikvah on erev Yom Kippur. One of the steps of the teshuvah process is to be purified in a mikvah.
Rebbe Mordechai of Chernobel zt’l said, "If someone comes to me without immersion in the mikvah, I can see all his bad deeds, even those he committed behind closed doors. But if he went to mikvah before coming to me, I don't see anything bad. The mikvah purifies him." (Raza d'Uvda). The mikvah erases the impression of sin.
Tzaddikim said, "If mikvah can turn a goy into a Yid (when he converts), imagine what it can do for a Yid!" We can add that if a person feels like a goy due to his sins, he should go to mikvah, because mikvah can turn a goy into a Yid.
The Mishnah states, כל שבים טהור, "All sea creatures are pure." The Iglei Tal (introduction) explains that when one goes to the mikvah, he is in a place where there are no impurities, so his tumah falls off.
The holy sefarim teach that when one goes to mikvah, he becomes like a newly born child. The Bnei Yissaschar (Tishrei, 4:11:31) explains that in a mikvah, one bends his head towards his chest and appears like an embryo. Therefore, when one comes out of the mikvah, he should consider himself as though he was just born, receiving a clean slate with which to serve Hashem.