[232-3] It is illuminating to recognize that subjective Justification, as well as objective Justification, has already taken place in Jesus Christ. Not only was the great divine act of righteousness fulfilled in the flesh of Jesus, in His Life and Death, but throughout His Life and Death Jesus stood in our place as our Substitute and Representative who appropriated the divine Act of saving Righteousness for us. He responded to it, yielded to it, accepted it and actively made it His own, for what He was and did in His human nature was not for His own sake but for our sakes. That is true of all that He did. He was the Word of God brought to bear upon man, but He was also man hearing that Word, answering it, trusting it, living by it—by faith. He was the great Believer—vicariously believing in our place and in our name. He was not only the Will of God enacted in our flesh, but He was the will of man united to that divine Will. In becoming one with us He laid hold upon our wayward human will, made it His very own, and bent it back into obedience to, and in oneness with, the holy Will of God. Likewise in Justification, Jesus Christ was not only the embodiment of God’s justifying act but the embodiment of our human appropriation of it. In that unity of the divine and the human, Justification was fulfilled in Christ from both sides, from the side of the justifying God and from the side of justified man—’He was justified in the Spirit’, as St. Paul put it. Justification as objective act of the redeeming God and Justification as subjective actualization of it in our estranged human existence have once and for all taken place—in Jesus.
[235-6] Jesus Christ was not only the fulfillment and embodiment of God’s righteous and holy Act…, but also the embodiment of our act of faith and trust and obedience toward God, He stood in our place, taking our cause upon Him, also as Believer, as the Obedient One who was Himself justified before God as His beloved Son in whom He was well pleased. He offered to God a perfect confidence and trust, a perfect faith and response which we are unable to offer, and He appropriated all God’s blessings which we are unable to appropriate. Through union with Him we share in His faith, in His obedience, in His trust and His appropriation of the Father’s blessing; we share in His justification before God. Therefore when we are justified by faith, this does not mean that it is our faith that justifies us, far from it—it is the faith of Christ alone that justifies us, but we in faith flee from our own acts even of repentance, confession, trust and response, and take refuge in the obedience and faithfulness of Christ—’Lord I believe, help thou mine unbelief.’ That is what it means to be justified by faith.
from “Justification: Its Radical Nature and Place in Reformed Doctrine and Life,” Scottish Journal of Theology, 13 no 3 (1960).