After Bnei Yisroel returned from the war with Midyan with the spoils, Moshe taught them the Halachos of Hagalas Keilim, Kashering the utensils through boiling water and fire. The parsha begins with the words Zos Chukas HaTorah (Matos 31:21). What is so special about the Halachos of Hagala that the Torah uses the strong words Chukas HaTorah?
Rav Moshe Feinstein answers that from the ability to make treif utensils kosher by boiling them we learn that a person who sullies his neshama with aveiros can also kasher it and cleanse it as if he never sinned. This is a pillar of the Torah, as it teaches people never to give up no matter how far they have gone away from the Torah.
Moreover we see that keilim used in fire need to be kashered with fire, libun. And keilim used in boiling water become kasher with boiling water. This teaches us that we must do tshuva the same way we did the aveira. If a person sinned with the burning desire of taava, so too must be the tshuva. And if the taava was less passionate then the tshuva can be on the same scale.