their justification received through faith...
The doctrine of Imputation set forth in the words on which we are commenting, involves the rejection of that theory of infusion of righteousness to which we have referred. The term imputation, as used in theology, does not mean simply a charge upon or against one, but rather the making of such a charge in terms of law and justice. We speak of the imputation of Adam's sin to his posterity, the imputation of man's sin to the second Adam, the imputation of Christ's righteousness to those who believe,—the imputation in each case being made in terms of the covenant of grace. Under the express conditions of that covenant, sin and righteousness respectively are regarded as of right belonging to the parties referred to therein. The ground of the sinner's justification is the work of Christ, the merit of which is attributed to us on condition of our believing in Him [see * Note below ]. The friend of another man's debtor says to his friend's creditor, put that debt to my account; when this proposal is accepted, the debt is imputed to me, who before this imputation was not chargeable with it, and he who was before a debtor is now in the state of one against whom the creditor can no longer advance a charge. Thus by the imputation of the sinner's guilt to Christ, the sinner who believes is justified.
That which is imputed to the sinner for his justification is described as Christ's perfect obedience to the law and satisfaction on the cross unto justice. This embraces the whole work of Christ, His active and passive obedience, His doing and suffering, His life and death. Like the changes of state in the believer enumerated in this section, these distinctions in regard to the work of Christ are not to be viewed as successive and temporally separable parts of Christ's life, but as two aspects illustrated throughout its entire course. He suffered in doing and He did in suffering. In His passion, which began in the first stages of His humiliation and was only consummated on the cross. He was not passive in the sense of merely submitting to a superior power : no man took His life from Him, but He laid it down,—not merely suffered it to be taken, for He had power to lay it down (John x. 18). The ground of our justification lies not in the death of Christ upon the cross alone. Christ's whole life of obedience unto death is that upon which we must depend for our justification.
The Sum of Saving Knowledge - With Introduction and Notes, 1871 - by Rev. John MacPherson M.A., page 127 - 128.
* Note: MacPherson explains what is meant by 'on condition of our believing in Him' on page 66:
But under the covenant of grace, God was dealing with a corrupt nature where selfishness and pride were already present. When faith was introduced in place of works as the condition of the covenant on man's side, there would be a danger of man's regarding faith as a work of his own upon which he might pride himself. It was necessary, therefore, in order to exclude boasting, to show those who are children of God by faith, and to make them remember that their faith was no work of their own, but a gift of God. Now, this is just another way of stating the doctrine of election. Those on whom God bestows the gift of faith are the chosen. It is His sovereign good pleasure alone that determines who are to receive this gift. It is the divine election which is the condition of our receiving His gift of faith. At the same time, so far as we are concerned, God's choice of us in His electing love, can become known only through our possession of that grace of faith which is the gift from God by which all His chosen are distinguished. In bestowing this gift He showeth mercy unto whom He will have mercy (Ex. xxxiii. 19; Rom. ix. 15).