Even suggesting a connection between cannibalism and Christian mass may sound extreme to believers, but just as the crucifixion of Jesus has a lot in common with older religious practices of human sacrifice, so too does the idea of transubstantiation — wine and bread becoming the blood and body of Jesus — have a lot in common with older religious practices of cannibalism. Crucifixion and mass are easier to understand if one understands the religious background of human sacrifice and cannibalism.
The concept of sacrificing something important to the gods or spirits was common in religions around the world. Usually, the more important the god or the request, the more important the sacrifice had to be. The most important thing which could be sacrificed was, usually, a human being. Typically, the person was sacrificed for the sake of the welfare of the entire community — to appease an angry god who had cursed the tribe, to plea for better crops, to ensure success in a coming battle, etc.
Ritual sacrifices, often connected to festivals surrounding a dying and resurrecting god, were important in Phoenician religion. Sacrifices of vegetable and animals were most common, but human sacrifices occurred in times of difficulty. The preferred human sacrifice was an innocent child who, as a vicarious victim, represented the most extreme act of propitiation possible and was probably intended to guarantee the future of the entire community.
For the Aztecs, consuming human flesh was a type of communion, establishing a sacred relationship between humans and gods. Because the people who were ritually sacrificed were "impersonators" of the gods, the Aztecs saw themselves as not consuming another human being, but as consuming a god. Such a role was regarded as an honorable and even coveted death — it had about the same status as a heroic death in battle. The sacrificial victim received a penitential liberation from this plane of existence, released to a new life with the gods.
Traditional Christian communion shares many of attitudes and beliefs with older forms of human sacrifice and cannibalism, but without all the blood and fuss. The idea of eating a god has been abstracted out and removed from the consumption of a real being and transformed into eating an allegedly "transformed" bit of bread. Few Christians would recognize the connections between communion and cannibalism, but perhaps if they did they would think a bit harder about what they are doing.