I will now briefly notice this union with Adam, &c. Whilst they were thus set up in Christ, and as such loved with an everlasting love, it was the pleasure of the Father that they should be partakers of flesh and blood, and therefore as rational creatures they were created in a natural head, Adam, and under the law; were in common with all his posterity, left to fall in him, and became subject to the curse of the law. While they were thus a part of the same fallen family of Adam involved in the same condemnation, they by the eternal purpose of God were distinguished from the rest of the human family - and were from the beginning chosen unto salvation; and predestinated unto the adoption of children, by Jesus Christ, and were given to Christ as His portion, as it is written: the Lord's portion is His people, Jacob is the lot of His inheritance. It was also necessary in order to their being brought into liberty as sons and receive the spirit of adoption, that they should be redeemed from under the law and also to be slain by it, that their union to it might be dissolved. For all this provision was made in the everlasting Covenant and the accomplishment thereof assigned to the Son and to the Holy Spirit in their respective offices. Their redemption could alone be accomplished by one who could fulfill the demands of the law and make it honorable in their behalf, and consequently alone by one who could be acknowledged by Divine Justice as standing in their law place. Christ being their Brother, their Head, their Husband in the everlasting Covenant, and in relation to the life therein set up, the right of redemption belonged to Him, and being not Himself under the law, He could take their law place and obey in their behalf, and be so accepted by Divine justice.
Samuel Trott - 1833
A true believer has for his starting place what the self-righteous religionist has for his goal. A true believer begins with the fact of salvation realized and the certainty of it by Christ. The self-righteous religionists is seeking to gain and maintain salvation by what he thinks he has been enabled to do. The difference between the two is saved (righteousness already established by Christ) or lost (seeking to establish a righteousness of your own).
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A Discourse on Justification
Thus it appears that the active and passive obedience of our Lord, or both these branches of His righteousness, were absolutely necessary to make us completely righteous in the sight of God and of His holy Law. And though these two parts of His obedience cannot be divided, yet they may be distinguished: divided they cannot be, inasmuch as that soul who is washed from sin in His blood (Revelation. 1:5), or by His passive obedience, is also made righteous by His active obedience; and whoever is made righteous by His active obedience is likewise made spotless by His blood. On which account the blessedness of a justified state is sometimes expressed by one part of His righteousness, and sometimes by the other. But though they cannot be divided, yet they may and must be distinguished: forasmuch as by His passive obedience we are more properly discharged from guilt and freed from the curse; and by His active obedience we are more properly made righteous and inherit the blessing. And both these parts of our Lord's obedience make up that one righteousness of His whereby we are justified in the sight of God; or, which is the matter of a sinner's justification before God.
And as the complete obedience of Christ in both its parts is the matter of justification, or of the justifying righteousness of a sinner before God; so it stands alone, as such, in its own comprehensive glory, exclusive of all the creature's works, whether before or after is regeneration by the Spirit of God. As, Even as David also describeth the blessedness of the man unto whom God imputeth righteousness without works (Romans. 4:6). This righteousness which is here said to be without works, is the obedience of Jesus Christ; which is the justifying righteousness of a sinner; and is so complete in itself that nothing can be added to it to make it more so. All the works of the creature since the Fall are imperfect; and therefore utterly unfit to be its justifying righteousness before God, either in whole or in part. It is impossible that an imperfect obedience can make the person that performs it perfectly righteous; and such is the infinite purity of God's nature and the strictness of His justice, that He can accept nothing for righteousness that is not perfectly conformed to the rule of it in His holy Law. And therefore our own obedience cannot be the whole of our righteousness before God. Nor can it be any part of it; because that which is wholly imperfect can be no part of perfection.