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
8-10 - Communion brought about by the Holy Spirit
8. We lost communion with God in our sin. We must be restored to it, to live. This has begun, body and soul, in believers - John 6:48, 51-2, 55-6.
9. We must partake of Christ's body and blood to "aspire to heavenly life." We see this in Eph 1:23 (The church is the body of Christ); 4:15-16 (He is the head; we are the body); 1 Cor 6:15 (our bodies are members of Him); Eph 5:30 ("we are members of His body, of His flesh and of His bones"). "It would be extreme madness to recognize no communion of believers with the flesh and blood of the Lord" when Paul marvels at the mystery, rather than explaining it.
10. The flesh and blood of Christ feed our souls, in the Supper. This only happens by union. The Spirit "unites things separated in space." This is why 1 Cor 10:16 calls the cup a "participation in His blood." It isn't just a figure of speech. There is symbolism, but also the reality for those who believe. Whenever we see the sacraments we should "be persuaded that the truth of the thing signified is surely present there." Why would He give you the symbol, and not the reality?