Book 4 of 4 - External Means by which God Invites Us into the Society of Christ and Holds Us Therein
Chapter 17 - The Lord's Supper; what it brings to us
32-34 - True nature of Christ's corporal presence
32. I don't claim to understand His presence and feeding of our souls with His body. "I rather experience than understand it.... I reject only absurd things" about it. "From the substance of His flesh Christ breathes life into our souls."
33. The inquisitive demand an exaggerated mode of Christ's presence in the bread itself, but the Spirit unites us with Him. If He is present physically and automatically in the bread, then unbelievers could eat and partake of Christ without faith, just by eating bread. This can't happen. Eating and drinking judgment against oneself at the table (1 Cor 11:27) doesn't happen by eating, but by disregarding the Lord while eating.
34. Who receives the sacrament? Augustine said he "who eats with the heart, not who presses with the teeth." This is because the wicked can't be members of Christ - 1 Cor 6:15. Augustine again: "In the elect alone do the sacraments effect what they symbolize."