John Gill: Scripture shows that we are justified by the imputed righteousness of Christ, not a righteousness that is our own

And now, that it may appear that we are justified by the righteousness of Christ imputed to us, observe,

1. That we are in our own persons ungodly, who are justified, for God justifieth the ungodly; (Rom. 4:5) if ungodly, then without a righteousness, as all Adam’s posterity are; and if without a righteousness, then if we are justified, it must he by some righteousness imputed to us, or placed to our account; which can be no other than the righteousness of Christ.

2. We are justified either by an inherent, or by an imputed righteousness; not by an inherent one, because that is imperfect, and nothing that is imperfect can justify us. Besides, this is a righteousness within us, whereas the righteousness by which we are justified is a righteousness without us; it is unto all, and upon all them that believe. (Rom. 3:22) And, if we are not justified by an inherent righteousness, then it must be by an imputed one, because there remains no other.

3. The righteousness by which we are justified is not our own righteousness, but the righteousness of another, even the righteousness of Christ: That I may be found in Christ, says the apostle, not having mine own righteousness, which is of the law, but that which is through the faith of Christ. (Phil. 3:9) Now, the righteousness of another cannot be made ours, or we he justified by it, any other way than by an imputation of it.

4. The same way that Adam’s sin becomes ours, or we are made sinners by it, the same way Christ’s righteousness becomes ours, or we are made righteous by it. Now, Adam’s sin becomes ours by imputation, and so does Christ’s righteousness, according to the apostle: As by one man’s disobedience many were made sinners, so, by the obedience of one, shall many be made righteous.

5. The same way that our sins became Christ’s, his righteousness becomes ours. Now our sins became Christ’s by imputation only; the Father laid them on him by imputation, and he took them to himself by voluntary susception; they were placed to his account, and he looked upon himself as answerable to justice for them. Now, in the same way his righteousness becomes ours: For he, who knew no sin, was made sin for us, that we might be made the righteousness of God in him, 2 Corinthians 5:21.

(John Gill, THE DOCTRINE OF JUSTIFICATION, BY THE RIGHTEOUSNESS OF CHRIST, STATED AND MAINTAINED)