I think if there is one area that is misunderstood today in theology it is the relationship between justification and union with Christ. There are a number of misunderstanding but let me highlight a few.
(1) There is a common misunderstand amongst NT scholars of the relationship between union with Christ and justification in Reformed Orthodoxy. Some of the NT scholars seem to imply at times if they are the first to see the connection.
(2) This misunderstand is then compounded by a failure to see how the imputation of Christ's righteousness is essential. It is of course distinct but inseparable from the imparted righteousness that comes through sanctification.
(3) This can lead to readings of the text that collapse Romans 6 into Romans 3-5 or vice versa. There is a minimization then of the legal verdict that is reckoned to us in justification as Christ's resurrection is His justification and that verdict is declared upon us. This verdict flows from the believer's union with Christ. Yet this verdict rendered is wholly distinct from the transformation of righteousness in our life that develops as a Christian--e.g. our sanctification. This sanctification flows to us by virtue of our union with Christ.
(4) Collapsing Romans 6 into Romans 3-5 or vice versa leads to a minimization or elimination of the verdict rendering power of the reckoning. This follows suit with a confusing of sociology and theology in Paul's doctrine. No doubt sociological issues drive the historical situation for Paul's response--yet we cannot fail to see the profoundly theological articulation that drives Paul's language and undergirds his world view. This failure leads to an over extension of the categories of ecclesiology and soteriology so that in some readings the former overtakes and overshadows the latter. While we cannot deny the ecclesiological implication of justification by faith, these ecclesiological implications flow out of our union with Christ. They precede from our justification. In a sense then ecclesiology and soteriology are also distinct but inseparable. Of course, both items are integrally connected to union with Christ. Those united to Christ are part of the one body--the new eschatological man being restored. Those united to Christ also receive his resurrection verdict imputed/reckoned onto them although they have in no way acted righteous as the Second Adam did. All this most properly flows from a robust covenant theology.
The Reformed Orthodox were not naive when it came to the relationship between union with Christ and justification. Over at ThomasGoodwin there is a great blog post highlighting this. It notes:
Union with Christ is the first saving grace in dignity. Owen calls it the “greatest, most honourable, and glorious of all graces” (148). Moreover, and I believe this is significant, union with Christ is first in respect of causality and efficacy, that is, “it is the cause of all other graces that we are made partakers of” (150). Those graces are adoption, justification, sanctififcation, perseverance, resurrection, and glory!Regarding justification, Owen argues that “our union with him is the ground of the actual imputation of his righteousness unto us” (150). So, imputation does not bring about union, but union is the context in which God imputes righteousness.Goodwin carries the same emphases in his own writings. He refers to union with Christ as the “fundamental constitution of a Christian” (5:350). In fact, union with Christ is the “first fundamental thing of justification, and sanctification, and all. The goal of the covenant, including its blessings, is to bring sinners into union with Christ. In specific relation to justification, Goodwin maintains that “all acts of God’s justifying us depend upon union with Christ, we having him, and being in him first, and then thereby having right to his righteousness” (8:406).Of course, for Goodwin, union with Christ is threefold; first, according to the terms of the eternal covenant (pactum salutis); second, by representation when he was on the cross; and, third, “an actual implanting and engrafting us into Christ” (8:406).
I for one think much discussion on justification needs to recover the elements of union with Christ. There needs to be clear thinking about the relationships that go on here. Yet it will do know good if we do not see the Biblically faithful articulations that have come before us. Sadly too many NT scholars may be good at historical exegesis--and we certainly have more first century historical sources than we did even a century ago to enhance this endeavor--but too often NT scholars have little time for a robust historical theology. Sadly, at times (though admittedly not always), this comes across as more than a slight dose of chronological snobbery.