This article, particularly section III-C, is one of the more enlightening articles I've read in recent memory on the subject of justification (written by a non-Calvinist, no less!). From what I've read, the relevance of a believer's union with Christ to justification is relatively under-emphasized. Understanding union to be the means by which Christ is able to act on our behalf as penal substitute and righteous representative - and hence, that union with Christ is a precondition for justification - is, I think, a powerful rebuttal to a vogue argument in Roman Catholic circles; viz. the Protestant view of justification implies nominalism. From the article:
The meaning of [justification] is clearly forensic. But the deeper question remains: is that forensic verdict an accurate and true assessment of the believer when united to Christ, or is it a nominal and putative designation of a recategorization within God’s mind alone? We are joined to Christ to the extent that we gain His identity in the eyes of justice. In that sense, the infused identity does make us subjectively righteous (when the subject is the whole man, consisting of both the man and Christ in union), but only insofar as we are joined to Christ and it is His righteousness–already accomplished in His human life–that is the only righteousness in view. However, when we are joined to Christ, we are not joined to the extent that either is lost in the other. The union is sufficient to make us one with Christ in the eyes of justice, but the righteousness that is now ours remains the righteousness that He lived and not any righteousness that we live out or accomplish–in that sense it is still an alien righteousness. This infused identity is the substance and reality which our prior justification had in view.
We can say we have been crucified with Christ, buried with Christ, and raised with Christ (Romans 6:1-11) when in us is the Spirit of Christ who unites us to Him (Romans 8:9-11). This is indeed an "infused identity" - we are, inherently and intrinsically, new creatures in Christ.
Obviously, this is not to suggest the ground of our justification is merited by anything we have accomplished, nor is it correct to speak of the basis of justification as infused righteousness. These are antithetical to the gospel. Rather, our union with Christ or infused identity is what makes justification compatible with realism, as the imputation (legal charge) of the righteousness merited by Christ to our account and the non-imputation of our sin due the penalty Christ paid for us is made possible by the fact we are in Christ and He in us. This union is real, not merely forensic. The Father therefore justifies believers who are ungodly in themselves yet - due to their union with Christ - righteous in Christ. Such justification is not a "legal fiction."
A few citations of Turretin as Reformed precedent for this view will be a fitting close:
...by our legal and mystical union, [Christ] becomes one with us, and we one with him. Hence he may justly take upon him our sin and sorrows, and impart to us his righteousness and blessings. So there is no abrogation of the law, no derogation from its claims; as what we owed is transferred to the account of Christ, to be paid by him.
...when the sin of another is said to be imputed to any one, it is not to be understood that the sin is, purely and in every sense, foreign to him but that, by some means, it pertains to him to whom it is said to be imputed; if not strictly his own, individually and personally, then (communiter) conjointly, on account of community between him and its proper author. For there can be no imputation of the sin of another, unless it is based upon some special union of the one with the other.