For Calvin, the importance of union with Christ, and the Holy Spirit’s role within union with Christ, is borne out especially in Calvin’s pneumatology. Calvin states that “faith is the principal work of the Holy Spirit,” and it is faith by which the believer is united to Christ. In fact, Calvin goes so far as to say that in II Thess. 2:13, Paul warns his readers that “faith itself has no other source than the Spirit.” Without the work of the Holy Spirit to bring about faith in the believer, the promise of salvation would never, in fact could never, penetrate into the hearts of men. Thus, it seems to follow at least in some sense that the Holy Spirit’s principal work in the lives of men is to unite them with Christ.
Regarding the faith with which we must embrace Christ, Calvin states that it is not a mere opinion, persuasion, or even mental assent to the history of the Gospels. Rather, it is a multifaceted knowledge by which the believer “recogni[zes] the divine goodness upon which our righteousness rests,” for faith is “a firm and certain knowledge of God’s benevolence toward us founded upon the truth of the freely given promise in Christ, both revealed to our minds and sealed upon our hears through the Holy Spirit.” It is this faith which the Holy Spirit works in us and by which the believer is united to Christ. Put another way, it is this faith wrought by the Holy Spirit that makes the benefits of Christ’s person and work ours. Because the Holy Spirit plays the principal role in working this faith in the believer, it can certainly be said that the Holy Spirit plays the principal role in uniting the believer to Christ, and thus plays a significant role in the salvation of the believer and the apprehension by the believer of Christ’s benefits. As Kolfhaus and Tamburello say, for Calvin “the Holy Spirit alone, and indeed alone through faith, engrafts us into Christ.”
The believer, in order to partake of Christ’s benefits, must be joined to Him. Calvin asserts that this union takes place through the secret working of the Spirit, for it is the Spirit who works faith within the hearts of believers, thus joining them to Christ. So Calvin: “It is true that we obtain this [union with Christ] by faith.” It is important to note that for Calvin, faith is not a work which men come to of their own accord, for it is the Spirit “by which we come to enjoy Christ and all His benefits.” It is clear that for Calvin, the Spirit works faith in the hearts of believers, and it is this Spirit-wrought faith which is the instrument which joins the believer to Christ, that the believer might partake of Christ’s benefits.
The persons of the Holy Spirit and Christ work in tandem for Calvin in the believer’s experience of union with Christ. Christ promises and sends the Spirit to His people (Jn. 14:17), and the Spirit bind believers to Christ, for “we know that [Christ] abides in us from the Spirit whom he has given us” (1 Jn. 3:24). Christ is the agent who brings believers into union with Him by baptizing believers in the Holy Spirit (3.1.4). Put another way, Christ sends the Spirit to baptize His people, working faith in them, and the Spirit brings Christ people back to Him in union with Christ.
That the Holy Spirit is involved in the believer’s union with Christ should not be in the least surprising, for Calvin finds himself in line with the best of church history when he makes redemption a Trinitarian endeavor. The Father, the Son, and the Holy Spirit are all intimately involved in the enterprise of redemption and, by extension, union with Christ. The Father elects for Christ a people, Christ gives a people back to the Father, and the Holy Spirit unites a people to Christ. Additionally, it is Christ who justifies His people, the Spirit who takes them up to Christ in the Supper, and the Father who finds them innocent on the Day of Judgment. All three persons, then, play integral yet distinct role in the salvation of God’s people for Calvin. It is against this Trinitarian backdrop that Calvin develops his doctrine of union with Christ.
 Ibid., 3.1.4.
 Ibid., 3.1.4.
 Ibid., 3.2.1.
 Ibid., 3.2.2.
 Ibid., 3.2.7. Calvin emphasizes other elements of faith as well, like that of “trust.”
 Mark A. Garcia, Life in Christ: Union with Christ and Twofold Grace in Calvin’s Theology (Colorado Springs, CO: Paternoster, 2008), 71.
 Calvin, Institutes, 3.1.1.
 Ibid., 3.1.1. It is worth noting that here, in seeking to answer why some men do not embrace the Gospel offered to them, Calvin also invokes the “secret energy of the Spirit.” Thus, the Spirit works faith within the believer, making faith a gift rather than a work, and connecting union with Christ to election, for only those are united to Christ within whom the Spirit works faith.
 Cited in Ibid., 3.1.4.
 Ibid., 4.17.31.
 Cf. especially J. Todd Billings, Calvin, Participation, and the Gift: The Activity of Believers in Union with Christ (New York, NY: Oxford University Press, 2007),, 105-143