Simul Iustus et Peccator - "At the same time righteous and a sinner".
Righteousness through Christ is called an "alien" righteousness because it did not generate from us. It is not our righteousness; it is His.
"We confess and acknowledge that the Law of God is most just, equal, holy, and perfect, commanding those things which, when perfectly done, can give life and bring man to eternal felicity; but our nature is so corrupt, weak, and imperfect, that we are never able perfectly to fulfill the works of the Law. Even after we are reborn, if we say that we have no sin, we deceive ourselves and the truth of God is not in us. It is therefore essential for us to lay hold on Christ Jesus, in His righteousness and His atonement, since He is the end and consummation of the Law and since it is by Him that we are set at liberty so that the curse of God may not fall upon us, even though we do not fulfill the Law in all points. For as God the Father beholds us in the body of His Son Christ Jesus, He accepts our imperfect obedience as if it were perfect, and covers our works, which are defiled with many stains, with the righteousness of His Son. We do not mean that we are so set at liberty that we owe no obedience to the Law--for we have already acknowledged its place--but we affirm that no man on earth, with the sole exception of Christ Jesus, has given, gives, or shall give in action that obedience to the Law which the Law requires. When we have done all things we must fall down and unfeignedly confess that we are unprofitable servants. Therefore, whoever boasts of the merits of his own works or puts his trust in works of supererogation [*believing we have sufficient merit of moral goodness apart from Christ], boasts of what does not exist, and puts his trust in damnable idolatry."
~ John Knox, Scots Confession 15