How can Christians feed on the body and blood of Christ when Christ is in heaven? Calvin answered that the Spirit of God unites to Christ. Christ does not descend into the bread, but the Spirit lifts the believer up to heaven: “What, then, our minds do not comprehend, let faith conceive: that the Spirit truly unites things separated in space” (4.17.10). For Calvin, the Supper is a spiritual communion, not in the sense that Christians commune with a disembodied Christ, but in the sense that, by the action of the Spirit, they commune with the whole, real Christ.
The promise of God and the work of the Spirit are certain to all who come to the table in faith (4.17.10). Faith does not create the promise or cause the Spirit to work. But only faith receives the blessing of the promise and the Spirit.
From: “Calvin, Worship, and the Sacraments: Institutes 4.13-19″ by W. Robert Godfrey, in Theological Guide to Calvin’s Institutes: Essays and Analysis, edited by David W. Hall and Peter A. Lillback; The Calvin 500 Series (Phillipsburg: P&R Publishing, 2008), p. 377.