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. (Philippians 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.
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.
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 voluntarily; 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.
By John Gill