Our Sages and commentators noted the parallels that exist between the states of purity and impurity and the cycle of sin, punishment and repentance. This can already be seen in the words of the Prophets who use terms of impurity to express sin, and terms of purity to express repentance and correction. The Sages went further and made the connection between the physical and material impurities mentioned in the laws of purity in Lev. 14-15 to specific sins and transgressions.
There is sometimes a direct connection between physical impurity and the actual sin; for example, leprosy (tzara'at) - which is a punishment for the sin of talking badly of others (lashon hara). The rehabilitation process for the sinner also includes social ostracization, and his purification is dependent upon his learning his lesson and repenting.
In other cases, the connection is not obvious; for example, impurity of childbirth or the menses. This kind of impurity does not come as a punishment for any specific sin committed by any woman; rather, it is connected to the sin of Adam and Eve: "I shall surely increase your sorrow and your pregnancy." (Gen. 3:16). This is part of the corruption caused by Hava's sin which necessitates a “tikkun” (repair) by way of purification processes for all future generations.
Then there are other areas in which it is difficult to pinpoint any sin; but even if the ensuing impurity is not the result of any particular sin, it nevertheless hints at overall failings of the soul. Even though details of the various ways to become pure again depend on the type of impurity involved, there is one principle action that must always be carried out :immersion in water. The more serious instances of impurity also necessitate bringing a sacrifice.Immersion in water lies at the very foundation of the concept of purity, and the sacrifice effects atonement: "and the Kohen shall effect atonement for him from his discharge" (15:15)
In his book "On Repentance" Rav J.B. Soloveitchik considers the difference between these two stages – purity and atonement – to be characterized as two different types of repentance. His words can be briefly explained as follows: Atonement involves correcting the sin and evil that has already been done. What you have broken – you must repair or pay for. Purity, however, involves banishing the root of what causes the sin from a person's soul. His soul becomes cleansed from the root of the sin, and there is good reason to believe that he will not sin again.