"From what has been stated, we may learn a satisfactory answer to this question, How shall man be just with God? How can an unrighteous person be accepted as righteous, by an infinitely righteous Judge? It can not possibly be on the ground of his own performances; for though he should even from the moment of his birth till that of his death, obey the law perfectly, it could never satisfy for the sin in which he was born. It is by the consummate righteousness of Jesus Christ received by faith, that he can be just in the eye of the law, or the sight of God.
"Hence learn, that the righteousness which is the ground of a sinner's justification, is not the believer's because it is imputed to him; but it is imputed to him because it is already his. In God's imputation of it, he reckons it to be what it is already, the believer's justifying righteousness. It is the believer's, in virtue of his legal union with Christ from eternity, and of his vital union with him in time.John Colquhoun, Sermon IX - Justification
"See also the glorious design of Jehovah in imputing the surety-righteousness of Jesus Christ to his spiritual seed. It is that you, believer, may live; may have a legal right to spiritual, temporal, and eternal life. Your sin was imputed to Christ that he might die: his righteousness is imputed to you that you may live."