The Chachamim teach (Ta’anit 27b and Megillah 31b) that when Hashem entered a covenant with Avraham Avinu and promised him that he and his children would inherit the Land of Israel, Avraham asked HaKadosh Baruch Hu, “Master of the Universe, perhaps, Heaven forbid, the nation of Israel will sin before You, and You will do to them as You did to the generation of the flood and the generation of the dispersion?” Hashem answered, “I will not do that to them.” Avraham said, “How will I know that You will not destroy their name?” Hashem responded, “Bring me three calves…” By that, He hinted to him that the institution of the korbanot will be witness to the eternal connection between Israel and Hashem. Therefore, even if sins should be found within the Jewish nation, it is only because of external influences – in their core the Jewish people are righteous and are all connected to Hashem. Therefore, by the offering of korbanot, an act which expresses Israel’s absolute devotion to Hashem, their sins will be atoned.
Avraham Avinu said before Him, “Master of the Universe, and what will be when the Temple will be destroyed; how will their sins be atoned?” HaKadosh Baruch Hu answered, “I have already instituted the Korbanot passages for them. Every time they recite them, I will consider it as if they are bringing an offering before Me, and I will forgive them for all their sins.”
Further, the Chachamim say that anyone who engages himself in the laws of the Chatat (sin) offering, it is as if he offered a Chatat; and anyone who engrosses himself in the laws of the Asham (guilt) offering, it is as if he offered an Asham. Likewise, when one engages himself in the laws of any of the korbanot, it is as if he brought those offerings (Menachot 110a).
The idea behind this is that every deed performed in the world possesses an inner soul. The soul of a mitzvah is the words of Torah that discuss that mitzvah. These ideas especially pertain to the korbanot, for the essence of the korbanot is to express our connection to Hashem. Therefore, when one cannot actually bring the offerings, the study of them is considered a substitute for their sacrifice (see also Maharal, Gevurot Hashem, chapter 8).