What does John 6:54 mean?

In this passage, Jesus repeatedly compares the means of salvation to eating His flesh and drinking His blood. The context for this analogy is Jesus' claim to be the "true bread from heaven," which is the sole source of salvation for the world (John 6:32–33). Several times in this section, Jesus will make this comparison, which has already deeply offended the crowd (John 6:43). Some cannot get beyond the shallow specter of cannibalism. Others will reject the idea of a Messiah who is not a conqueror (John 6:51). In the end, all but a small group will choose not to follow Jesus anymore (John 6:66).

The fact that Jesus is speaking in poetic terms here is explicitly proven later in His discourse. Therefore, this passage cannot be taken as proof of the doctrine of "transubstantiation:" that modern communion literally becomes the flesh and blood of Jesus. Christ, in His own words, says this is not the case (John 6:63).

Jesus also ties the importance of this claim to the same statement He made earlier: that those who believe will see eternal life (John 6:40). That was in the context of belief in the One sent by God (John 6:27–29). The people should have seen the parallels here, but they are too busy complaining (John 6:41) and fighting (John 6:52) to sense the deeper meaning.