What is the significance of the fact that believers are united to Christ? Reformed theologians have historically argued that there are a number of different aspects to our union with Christ.
For example, we are united to Christ in terms of our election “in him.” We were not indwelt by the Holy Spirit at this point and united to Christ by faith because we did not even exist except in the mind of God. Nevertheless, we are united to Christ in terms of the Father’s decision to elect individual fallen sinners and redeem them through His Son. Hence, in this sense, we are united to Christ in the decree of election.[/QUOTE]
for any who would like to read more;
http://www.ligonier.org/learn/articles/union-with-christ-in-pauls-epistles/
or another;
Union with Christ is at once a difficult and woefully neglected subject (the latter possibly explained by the former). Yet Sinclair Ferguson writes that union with Christ is “a doctrine which lies at the heart of the Christian life and is intimately related to all the other doctrines.… Union with Christ is the foundation of all our spiritual experience and all spiritual blessings.”11 And Murray observes that “Union with Christ is really the central truth of the whole doctrine of salvation, not only in its application but also in its once-for-all accomplishment in the finished work of Christ.”12 In addition, the respected theologian A. W. Pink introduces his work on union with this emphatic statement:
The present writer has not the least doubt in his mind that the subject of spiritual union is the most important, the most profound, and yet the most blessed of any that is set forth in the sacred Scriptures; and yet, sad to say, there is hardly any which is now more generally neglected. The very expression “spiritual union” is unknown in most professing Christian circles, and even where it is employed it is given such a protracted meaning as to take in only a fragment of this precious truth. Probably its very profundity is the reason why it is so largely ignored in this superficial age.13
Counselors must emphasize the doctrine of union with Christ because it incorporates two key issues essential to understanding change and struggle. First, union with Christ is an all-encompassing doctrine. “It embraces the wide span of salvation from its ultimate source in the eternal election of God to its final fruition in the glorification of the elect.”14 Second, it is the one doctrine that embraces the factors of what Christ has accomplished (the indicative) and what believers are commanded to do (the imperative). Moule says that the gospel begins “in the indicative statement of what God has done,” and before it goes on to the imperatives “to struggle” it confronts us with the imperative “to attach oneself (be baptized! be incorporate!).”15
Configured between what God has accomplished in Christ and what we are to do in obedience, and possessed with a scope that extends from eternity past to eternity future, union with Christ is an indispensable doctrine in understanding change and struggle in people’s lives.
http://learntheology.com/union-with-christ-the-implications-for-biblical-counseling.html
again;
Historically Reformed theologians have recognized that union with Christ is not merely one aspect of the order of salvation but is the hub from which the spokes are drawn. One can find such conclusions in the theology of Reformed luminaries such as John Owen, Herman Witsius, and Thomas Boston, to name a few. That union undergirds the whole of the order of salvation is evident from Paul's book-end statements
that we were chosen in Christ before the foundation of the world and that only those who are in Christ will be raised from the dead and clothed in immortality.
In fact, we may say that there are three phases of our union with Christ, the predestinarian "in Christ," the redemptive-historical "in Christ," the union involved in the once-for-all accomplishment of salvation, and the applicatory "in Christ," which is the union in the actual possession or application of salvation. These three phases refer not to different unions but rather to different aspects of the same union.
http://modernreformation.org/default.php?page=printfriendly&var1=Print&var2=7
