Gaffin on Union

If, then, we ask, what is the ground of the justification that is mine in union with Christ, the basis of my being justified in him, then, it seems, there are just three conceivable options in reading Paul: (a) Christ's own righteousness, complete and finished in his obedience culminating on the cross, the righteousness that he now is and embodies in his exaltation; (b) the union itself, the fact of the relationship, the existence the uniting bond, as such; or (c) the righteousness and obedience being produced by the transforming work of the Spirit in those in union with Christ. In short, in union with Christ, the ground of justification is resident either in Christ as distinct from the believer, in the bond between Christ and the believer itself, or in the believer as distinct from Christ.

It appears that the current readiness to dispense with imputation stems from taking either of the latter two factors just mentioned, whether or not intentionally, as in effect, the ground of justification. But neither is sustainable. The relationship as such, no matter how real and intimate, in distinction from the persons in that relationship, cannot be the basis of my justification. Clearly in Paul it is not a relationship as an entity, the relational bond in itself, but a person that justifies and saves, specifically the person of "the Son of God, who loved me and gave himself for me" (Gal 2:20). I suspect that position (b) above will inevitably gravitate to (c) in some form...

- Richard Gaffin, By Faith, Not by Sight: Paul and the Order of Salvation, pg. 51