“It is sometimes said that the merits of Christ cannot be imputed to us as long as we are not in Christ, since it is only on the basis of our oneness with Him that such an imputation could be reasonable. But 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 (or future) condition, but on that of a gracious imputation–a declaration which is not in harmony with the existing condition of the sinner. The judicial ground for all the special grace which we receive lies in the fact that the righteousness of Christ is freely imputed to us.” (Systematic, p 500)By God's election we are legally united to Christ our Surety and Mediator. Thus his death is our death. His obedience is our obedience. Upon his resurrection God imputes Christ's finished work to the elect. It is Jesus, who was delivered up for our trespasses, and was raised for our justification (Rom. 4:25). The declaration of Justification is then brought to us in the gospel and made ours when by the work of the Holy Spirit through God's effectual calling we are made alive in Christ, given the grace of saving faith by which we are united spiritually with Christ and justified in Him. The imputation of Christ's passive and active obedience is the legal (forensic) ground of our effectual calling. We receive that judgment or status, that legal justification through faith upon hearing the gospel. Westminster Larger Catechism:
Q. 57. What benefits hath Christ procured by his mediation?
A. Christ, by his mediation, hath procured redemption, with all other benefits of the covenant of grace.
Q. 58. How do we come to be made partakers of the benefits which Christ hath procured?
A. We are made partakers of the benefits which Christ hath procured, by the application of them unto us, which is the work especially of God the Holy Ghost.
Q. 59. Who are made partakers of redemption through Christ?
A. Redemption is certainly applied, and effectually communicated, to all those for whom Christ hath purchased it; who are in time by the Holy Ghost enabled to believe in Christ according to the gospel.
Q. 70. What is justification?
A. Justification is an act of God's free grace unto sinners, in which he pardoneth all their sins, accepteth and accounteth their persons righteous in his sight; not for anything wrought in them, or done by them, but only for the perfect obedience and full satisfaction of Christ, by God imputed to them, and received by faith alone.
Q. 71. How is justification an act of God's free grace?
A. Although Christ, by his obedience and death, did make a proper, real, and full satisfaction to God's justice in the behalf of them that are justified; yet inasmuch as God accepteth the satisfaction from a surety, which he might have demanded of them, and did provide this surety, his own only Son, imputing his righteousness to them, and requiring nothing of them for their justification but faith, which also is his gift, their justification is to them of free grace.
Q. 72. What is justifying faith?
A. Justifying faith is a saving grace, wrought in the heart of a sinner by the Spirit and Word of God, whereby he, being convinced of his sin and misery, and of the disability in himself and all other creatures to recover him out of his lost condition, not only assenteth to the truth of the promise of the gospel, but receiveth and resteth upon Christ and his righteousness, therein held forth, for pardon of sin, and for the accepting and accounting of his person righteous in the sight of God for salvation.
Q. 73. How doth faith justify a sinner in the sight of God?
A. Faith justifies a sinner in the sight of God, not because of those other graces which do always accompany it, or of good works that are the fruits of it, nor as if the grace of faith, or any act thereof, were imputed to him for his justification; but only as it is an instrument by which he receiveth and applieth Christ and his righteousness.