In Rambam's view, only because Am Yisrael had grown accustomed to the pagan practice of animal sacrifice did the Almighty feel compelled, as it were, to mandate such a system for Benei Yisrael. Many writers have pointed to the sacrifices of Kayin and Hevel as conclusive evidence against the controversial view of the Rambam. But why did God accept Hevel's sacrifice and not Kayin's? Are sacrificial offerings enough?
Parashat Bereishit records the first offering of sacrifices mentioned in the Torah - those of Kayin and Hevel. (Several Midrashic sources indicate that Adam had offered sacrifices even earlier.) Many writers have pointed to these sacrifices as conclusive evidence against the controversial view of the Rambam concerning the nature of sacrifices according to Jewish theology (see Moreh Nevukhim, 3:46). The Rambam claimed that God required the sacrificial order merely as a necessary concession to the pervading religious concept of sacrificial ritual. Only because Am Yisrael had grown accustomed to the pagan practice of animal sacrifice did the Almighty feel compelled, as it were, to mandate such a system for Benei Yisrael. The notion of sacrificial worship had become so ingrained that God could not expect the nation to adopt a religious system without it. This approach of the Rambam has met the harsh criticism of many later writers, most famously the Ramban, in his commentary to the beginning of Sefer Vayikra (1:8).
Among the more compelling proofs brought against the Rambam's position is the offerings of Kayin and Hevel, who lived before pagan ideas ever surfaced, and hence before the proliferation of the notion of sacrificial worship. The Ramban, and others, therefore argue for the essential value of korbanot, that they possess inherent religious meaning and significance, and are not merely a necessary capitulation to pagan practice.
Many attempts have been made to defend the Rambam's view. The Abarbanel, in his introduction to Sefer Vayikra, does not accept the Rambam's approach but nevertheless advances a possible explanation to respond to the challenges raised against it. He claims that the Rambam accepts the view of the Ramban and others that korbanot possess inherent significance; even before the pagans utilized sacrificial rituals as part of their idolatrous worship, sacrifices were a valuable means of achieving closeness to God.
What troubled the Rambam is why God chose specifically this means of achieving closeness. However one explains the efficacy of sacrifices, there are other ways of reaching the same goal far more effectively - more obviously, through prayer. Why, then, did the Almighty ordain an elaborate and intricate system of korbanot, without instituting a parallel system of prayers? (According to the Rambam, tefila is required by Torah law, but only any brief utterance over the course of a day, as opposed to the specific, liturgical ritual established by Chazal.) It is this question the Rambam seeks to resolve with his theory; once sacrifices had become such an integral part of mankind's perception of religious ritual, it earned a prominent role within the broader system of Torah and mitzvot. (Another, emphatic defense of the Rambam's position is launched by Rav Barukh ha-Levi Epstein, in his "Tosefet Berakha," beginning of Sefer Vayikra.)
In conclusion, we cite here an important observation by Rav Shimshon Refael Hirsch in his commentary to the story of Kayin and Hevel. He indeed sees the offerings of Kayin and Hevel as conclusive proof against the Rambam's theory (though he does not mention the Rambam by name). He adds, however, that here, in the Torah's first mention of korbanot, we see not only evidence of their inherent value and efficacy, but also that this efficacy is not absolute. Kayin and Hevel both brought offerings, but only one was accepted by God. This, too, reflects a critical point regarding our perspective on the korbanot: ritual offerings to God are effective only insofar as they are accompanied by sincere religious devotion.
Regardless of how one understands why God refused Kayin's offering (Rav Hirsch himself advances an interesting approach), clearly there was something lacking in his sincerity. Thus, the proof to the inherent worth of korbanot also proves the inherent limitation of korbanot - and of all religious ritual. Rituals do not work magic; we must perform them with a genuine desire to serve the Creator, observe His laws, and follow the path of Godliness.