The last couple days the Gospel readings have come from last part of John 6, otherwise known as the Bread of Life Discourse. Jesus speaks the truth all the time, but this is where the rubber hits the road. This short section separates the true believers from the hangers on.
Jesus repeatedly says that he is the Bread of Life, you must eat his Flesh and drink his Blood to attain eternal life. Jesus never said that he is "like the Bread of Life." No, he IS the Bread of Life. This sounded like cannibalism to many of his disciples. Jesus' words were so hard to accept that they turned back to their old lives and no longer followed Jesus (see verse 66).
There's another reason why this teaching was hard for many to hear. Jesus didn't simply say, "I am the Bread of Life." He said, "I AM the Bread of Life." The same I AM that speaks to Moses in the Book of Exodus - I AM THAT AM. To many ears it sounded like Jesus was blaspheming! But what it means is that Jesus isn't some cool dude, or simply a prophet, or even the messiah-king. Jesus IS God. This is Immanuel, God with us, telling us that we must eat his Flesh and drink his Blood. In other words, welcome to the Mass.