I've thought a lot over the past several months about Calvin, the Reformed tradition, and what role the doctrine of union with Christ has to play in the Christian life. You can find the Calvin-specific posts here, here, and here, and some different yet not unrelated posts here and here.
Okay; this will actually be my last post on Calvin's Union with Christ stuff (probably). Christ’s benefits as expounded in Calvin’s doctrine of union with Christ encompass the whole of Christian experience. At the beginning of the Christian life, the believer comes to possess both “cleansing and justification” by virtue of the Holy Spirit’s binding of the believer to Christ. Not only that, but the believer undergoes a complete and total identity change, as they are no longer reckoned to be a sinner but rather an heir with Christ, and an owner of all that Christ is an owner of. It is only through the Spirit’s joining of the believer to Christ that the believer can “taste either the fatherly favor of God or the beneficence of Christ.”
Most importantly, by being joined to Christ, the Holy Spirit applies to the believer what Calvin calls a duplex gratia, or double grace. For Calvin, the believer receives from Christ the forensic grace of justification and the transformative grace of sanctification. First, in justification, the believer is “reconciled to God through Christ’s blamelessness,” that they may have “in heaven instead of a Judge a gracious Father.” It is the benefit of justification, conferred to the believer by virtue of the believer’s union with Christ, which changes their judicial position before the Father. It changes the very nature of the relationship between the person and God, for Calvin, for justification by Christ propitiates God and expiates the sinner, allowing God to remain just while still punishing sin in Christ.
For Calvin, justification is by nature twofold: it includes the believer’s being reckoned before God as righteous, and the believer’s being accepted on account of that very righteousness. No unjustified sinner with any unrighteousness can ever find any favor in the eyes of the God of Israel. For Calvin, this justification is intimately bound up with the doctrine of union with Christ. Calvin says regarding justification, “He who…grasps the righteousness of Christ through faith, and clothed in it, appears in God’s sight not as a sinner but as a righteous man.” It is important to note that Calvin’s language is not that which would describe a cold, bare transaction between two disinterested parties. Rather, the relationship between the believer and Christ is so intimate that the believer is said to be “clothed in [the righteousness of Christ].” Since Christ’s righteousness is inseparable from His person, it is clear that in this statement Calvin urges the believer, if he is to be justified, to find in Christ his refuge and protection. Indeed, Burger helpfully points out that the image of “clothing oneself with Christ” is one of Calvin’s favorite and most frequently used images to refer to the “mystical union” of the believer with Christ. Yet, the believer can only find himself in Christ and clothed in His righteousness if, through faith wrought by the Holy Spirit, he is united to Christ.
This justification, in the application of which the Holy Spirit plays a large role, consists in both positive and negative dimensions for Calvin. On the one hand, justification is negative because it consists in the canceling, or the forgiveness, of sins. The believer is justified because Christ, in His union with the believer, pays the penalty for his sin on the Cross. The debt owed to God is paid by Christ, and by virtue of the believer’s union with Christ, the payment of that debt is credited to the believer. On the other hand, justification has within itself a positive element, because it is not only the canceling of the believer’s debt to God, but it is also crediting of Christ’s righteousness to the believer’s account. It is important to observe that in this positive element of justification, Calvin is careful to avoid saying that we possess the righteousness of Christ in ourselves in a sort of immediate ontological transformation. In fact, Calvin states that “that ‘we are made righteous by Christ’s obedience [Rom. 5:19ff]—could not stand unless we are reckoned righteous before God in Christ and outside of ourselves.” Worth noting is Calvin’s remark that the believer can only possess this righteousness if he is “in Christ.” The only way for the believer to find himself “in Christ,” and thus “outside of [himself]” and in possession of Christ’s righteousness is to have been united to Christ by the Holy Spirit.
The second aspect of the duplex gratia which the believer receives upon union with Christ is the transformative grace of sanctification. In sanctification, the believer grows in holiness through the power of the Holy Spirit and is conformed to the image of Christ. Sanctification always accompanies justification for Calvin, for justification without sanctification cannot exist since Christ cannot be “divided into pieces.” Similar to justification, Calvin’s exposition of sanctification enjoys an intimate connection with the doctrine of union with Christ, for it is through that same union by which the believer is justified that the believer is also sanctified. Indeed, it is the believer’s very own “sharing in Christ” that is the source of God’s gift of transformative sanctification. Ronald Wallace even remarks that “our conformity to Christ depends on union rather than imitation…It is within the relationship of union with Christ that we are exhorted to imitate Christ as our example.” Calvin’s grounding of sanctification in union with Christ rather than in a simple cause-and-effect relationship with justification relates sanctification immediately to Christ and to the Holy Spirit, making the source of sanctification the “life-giving Spirit” (1 Cor. 15:40ff) rather than simply the motivation of gratitude which would result from a simple cause-and-effect approach to sanctification. Thus, Calvin can say without reservation that the Holy Spirit, in uniting the believer to Christ, is called the “Spirit of sanctification” because “he is also the root and seed of heavenly life in us.” The case is strong, then, that sanctification in the life of the believer finds its source for Calvin in the doctrine of union with Christ.
 Calvin, Institutes, 3.1.1.
 Ibid., 3.1.2.
 Ibid., 3.11.1.
 Ibid., 3.11.2.
 Ibid., 3.11.2.
 Hans Burger, Being in Christ: A Biblical and Systematic Investigation in a Reformed Perspective (Eugene, OR: Wipf and Stock, 2009), 145.
 Calvin, Institutes, 3.11.4. Calvin cites 2 Cor. 5:18-20 in support of this point.
 Ibid., 3.11.4. In support of this, Calvin cites 2 Cor. 5:21.
 Ibid., 3.11.4
 Ibid., 3.16.1.
 Ibid., 3.16.1.
 Ronald S. Wallace, Calvin’s Doctrine of the Christian Life (Tyler, TX: Geneva Divinity School Press, 1959), 47.
 Calvin, Institutes, 3.1.2.