Within Calvin's Institutes, there are many themes. Calvin seeks to address many points of doctrine, but one stands out over and above the rest to me. Union with Christ, it might be said, is a motif which helps organize the rest of Calvin's work.. Calvin refers to the believer’s union with Christ as a “mystical union.” Union with Him is a very real identification with the person of Christ. Calvin makes clear that the believer does not “behold him from afar,” but rather “put[s] on Christ and [is] engrafted into his body.” Moreover, the believer is not simply engrafted into Christ, but also Christ indwells the believer in a sort of mystical perichoresis.
For Calvin, union with Christ is that by which the believer “receives those benefits which the Father bestowed on his only-begotten Son—not for Christ’s own private use, but that he might enrich poor and needy men.” By virtue of union with Christ, all that belongs to Christ belongs to the believer. Not only that, but each and every thing which Christ earned in His life and death, He earned specifically so that the believer might benefit from it. That Christ’s benefits are given to those united to Him is not simply a happenstance benefit of a solipsistic incarnation, life, death, and resurrection, but rather Christ was incarnated, lived, died, and was raised for the express purpose that men would be joined to Him and possess His benefits. In a word, given Calvin’s definition of union with Christ, the Incarnation is an outwardly focused event—Christ comes to Earth specifically to join men to Himself for their salvation.
The joining of men to Christ is critical in regard to the salvation of men for Calvin. As long as “Christ remains outside of us, and we are separated from him, all that he has suffered and done for the salvation of the human race remains useless and of no value to us.” The joining of the believer to Christ can be no less than the mystical union of the believer and Christ if the believer is to enjoy His benefits. Indeed, a very real indwelling of the believer in Christ is necessary for the believer to cease being “outside” of Christ. Christ must “dwell within us,” for “all that he possesses is nothing to us until we grow into one body with him.”
As foundational as union with Christ is in the Institutes, it presupposes at an even more foundational level the dire state of man after the Fall of Genesis 3. Man only needs to be united with Christ because in Adam, he lost communion with God in the Garden. It is the incarnation of Christ which ultimately is given for the salvation of God’s people from the perdition due every man for his sin in Adam and his failure to fulfill the Law in any way. However, the Incarnation as a discrete historical event does not save men. There must, for Calvin, be a way for the believer to appropriate Christ’s Incarnation, Life, Death, and Resurrection, for Christ’s life must belong to the believer if the believer is to have any hope of salvation at the final judgment. It is the apprehension of Christ’s Incarnation, given for the salvation of men to the glory of God, by “firm faith” that men might “embrace this mercy and rest in it with steadfast hope.” Faith, then, is the instrument by which fallen and sinful men grasp hold of the salvation offered in Christ’s Gospel. Indeed, Union with Christ is the antidote to man's tragic Fall.
 John Calvin, Institutes of the Christian Religion, ed. John T. McNeill, trans. Ford Lewis Battles (Philadelphia, PA: The Westminster Press, 1960), 3.11.10.
 Calvin, Institutes, 3.11.10.
 Calvin, Institutes, 3.11.10. This is not to say that Calvin is a mystic of any broad sort. Rather, it is simply to say that within Calvin’s doctrine of union with Christ, a certain reciprocity exists between the believer’s indwelling of Christ and Christ’s indwelling of the believer.
 Calvin, Institutes, 3.1.1.
 Calvin, Institutes, 3.1.1. Here, Calvin sets forth the “engrafting” imagery of Rom. 11:17 and the clothing imagery of Gal. 3:27.
 Calvin, Institutes, 2.12.7
 Calvin, Institutes, 3.2.1.