There are essentially two different levels of teshuvah: The power of teshuvah on the level of “before Creation”, and the power of teshuvah on the level of “after Creation”.
Chazal state that Hashem created teshuvah even before He created the world, and this is the power of teshuvah of “before Creation”, which is a deeper and more powerful kind of teshuvah, for it is from a dimension in which there was no possibility of sin yet. When a person uses the power of teshuvah of “before Creation”, he is accessing the root of becoming a totally “new being”, just as in the beginning state of Creation; this enables him to gain a “pure heart”, enabling him to return to a state in which he not only repents from the sin, but that the sin is completely erased, for he has returned to the point of before the sin.
Doing teshuvah on a level of “after Creation” can also purify a person, but it cannot bring a person back to the original pure state as if the sin was never here before.
