We may summarize this current trend in some portions of the Reformed community as a rejection of the imputed righteousness of Christ because the filial aspect of God's relationship to his people is primary. Justification of the believer, therefore, is reached, not through imputation, but through union with Christ. (p269)Fesko notes that the filial and forensic categories are not antithetical, but complementary and interconnected (p274). He cites examples such as Paul's language of adoption, where we see a legal concept tied to a relational one. To pit the legal against the filial aspects of union, seems to be an analogous argument to that of the man who won't marry his girlfriend because he "doesn't need a piece of paper to prove his love."
Other, more moderate Reformed re-workings of union, includes the work of men like Richard Gaffin of Westminster Theological Seminary Philadelphia, who attempt to make mystical union with Christ the centre of Paul's theology. A recent video by WTS Associate Systematic Theology Professor Lane Tipton affirmed this position. He was concerned that some make the forensic aspect of salvation so central that it "eclipse(s) the person of Christ. You first possess Christ and then in Christ you are justified." (about 3minutes and 10 seconds in). Fesko quotes Berkhof:
...this view fails to distinguish between our legal unity with Christ and our spiritual oneness with Him, and is a falsification of the fundamental element in the doctrine of redemption, namely, of the doctrine of justification. Justification is always a declaration of God, not on the basis of an existing condition [emphasis mine], but on that of a gracious imputation - a declaration which is not in harmony with the existing condition of the sinner. ( Berkhof, ST p452 quoted on p278)Fesko then quotes Calvin (who seems anachronistically to disagree with Tipton's take on Calvin!):
By faith we not only acknowledge that Christ suffered for us and rose from the dead for us, but we receive Him, possessing and enjoying Him as He offers Himself to us. This should be noted carefully. Most consider fellowship with Christ and believing in Christ to be the same thing: but the fellowship we have with Christ is the effect of faith. (John Calvin Ephesians 3:17, quoted on p279)Why is it important to see the forensic aspect of union as foundational? What's the problem with pitting mystical union first? Isn't it quibbling over words? The problem with it is that it subtly undermines the Reformed doctrine of God justifying the ungodly. God doesn't justify someone because they are in mystical union with Christ. That would be to justify on a pre-existing condition of the sinner. No, God justifies the ungodly by imputing righteousness to them. God "calls into existence the things that do not exist (Rom. 4:17)". I'll leave the last word with Fesko:
This conclusion [i.e. the priority of forensic union] plays a key role in the differences between the Reformed and Roman Catholic understanding of justification and salvation. For the Reformers, salvation came about because of the declaration of righteousness in justification, whereas for the Roman Catholic Church, even to this day, it is inverse: transformation precedes the declaration, the transformative is the judicial ground for the forensic. (p280)