James Buchanan: Christ's righteousness is the only and all sufficient righteousness, on the grounds of which we can be justified before God

That righteousness [which is the ground of a sinner's Justification] is called in Scripture "the righteousness of God"; "the righteousness of Christ"; the "righteousness of One"; "the obedience of One"; the "free gift unto justification of life"; "the righteousness which is of" or "by" or "through faith"; "the righteousness of God without the law"; and "the righteousness which God imputes without works."

It will be found that, while these various expressions are descriptive of its different aspects and relations, they are all employed with reference to the same righteousness -- that there is one righteousness in which they all find their common center, as so many distinct rays converging towards the same focus, while each retains its distinctive meaning -- and that there is no other righteousness to which they can all be applied or in which they can find their adequate explanation.

... By His [Christ's] vicarious sufferings and obedience, He fulfilled the Law both in its precept and its penalty and is now said to be 'the end of the law for righteousness to every one that believeth,' while His righteousness is identified with 'the righteousness of God,' to which the unbelieving Jews refused to 'submit themselves' and contrasted with 'their own righteousness' which they 'went about to establish,' 'as it were by the works of the law' (Rom 10:3-4)."

-Dr. James Buchanan, "The Immediate and Only Ground of Justification: The Imputed Righteousness of Christ"

Matthew Henry on Galatians 5:4

"Christ is become of no effect unto you, whosoever of you are justified by the law; ye are fallen from grace" (Galatians 5:4).

"All those who seek to be justified by the law do thereby render Christ of no effect to them. By building their hopes on the works of the law, they forfeit all their hopes from him; for he will not be the Saviour of any who will not own and rely upon him as their only Saviour." (Matthew Henry, Commentary on the Whole Bible, Galatians 5:4.)

Archibald Alexander: Suffering the penalty is only half the requirement for justification, and Christ satisfied all

To suppose that suffering the penalty is an equivalent for obedience, and entitles to the same rewards is extremely absurd. It would be to suppose that Jehovah who loveth righteousness and hateth iniquity, would be as well pleased with sin, accompanied with its due punishment, as with perfect obedience to his own most holy law. The enduring a penalty in his own person, or by another, never can entitle any one to any thing else than exemption from that which he has already endured. …

If a surety would secure the inheritance for him, he must obey the law in his stead, as well as suffer its penalty. Hence it appears evident, that justification includes more than merely the remission of sins, or it would be no justification; and although pardon is included in justification; yet the transaction receives this denomination not from the forgiveness of sin, but from the imputation of righteousness, by which the believer is constituted righteous; and by which a title to eternal life is procured for him by the merit of his surety.

Justification, therefore, is not merely the forgiveness of sin, but in addition to this, a declaration that the justified person has a right to the blessings promised. He not only obtains deliverance from the sentence of condemnation, but instantly is constituted an heir of God, a joint-heir with Christ in the heavenly inheritance.

-Archibald Alexander, “A Treatise on Justification by Faith”

A.W. Pink: Transgressing the Law and paying the penalty does not remove the positive obligation for perfect righteousness

The one who was justified upon his believing sustained a twofold relation unto God: first, he was a responsible creature, born under the law; second, he was a criminal, having transgressed that law—though his criminality has not canceled his obligation to obey the law any more than a man who recklessly squanders his money is no longer due to pay his debts. Consequently, justification consists of two parts, namely, an acquittal from guilt, or the condemnation of the law (deliverance from Hell), and the receiving him into God’s favour, on the sentence of the law’s approval (a legal title to Heaven). ... Against this it has been objected, "The law requires no man to obey and die too." To which we reply in the language of J. Hervey (1750), "But did it not require a transgressor to obey and die? If not, then transgression robs the law of its right, and vacates all obligation to obedience. … It is not sufficient that the believer stand before God with no sins upon him—that is merely negative. The holiness of God requires a positive righteousness to our account—that His Law be perfectly kept. But we are unable to keep it, therefore our Sponsor fulfilled it for us.” –A.W. Pink, “Justification” (ch. 5)

Shedd and Hodge: Christ satsified the penalty and precept of the Law for His people

“But the law requires present and perfect obedience, as well as satisfaction for past disobedience. The law is not completely fulfilled by the endurance of penalty only. It must also be obeyed Christ both endured the penalty due to man for disobedience, and perfectly obeyed the law for him; so that He was a vicarious substitute in reference to both the precept and the penalty of the law. By his active obedience He obeyed the law, and by his passive obedience He endured the penalty. In this way his vicarious work is complete.” –William Shedd, "History of Christian Doctrine," quoted by Charles Hodge in “Systematic Theology,” vol. 3

“As Christ obeyed in suffering, his sufferings were as much a part of his obedience as his observance of the precepts of the law. The Scriptures do not expressly make this distinction, as they include everything that Christ did for our redemption under the term righteousness or obedience. The distinction becomes important only when it is denied that his moral obedience is any part of the righteousness for which the believer is justified, or that his whole work in making satisfaction consisted in expiation or bearing the penalty of the law. This is contrary to Scripture, and vitiates the doctrine of justification as presented in the Bible.” –Charles Hodge in “Systematic Theology,” vol. 3

"Christ sustained no other relation to the law, except so far as voluntarily assumed, than that which God himself sustains. But God is not under the law. He is Himself the primal, immutable, and infinitely perfect law to all rational creatures. Christ’s subjection to the law therefore, was as voluntary as his submitting to the death of the cross. As He did not die for Himself, so neither did He obey for Himself. In both forms of his obedience He acted for us, as our representative and substitute, that through his righteousness many might be made righteous." –Charles Hodge in “Systematic Theology,” vol. 3

John Robbins: Works are not the condition, ground, or instrument of salvation, but rather salvation is the condition or ground of obedience

"But the Holy Spirit speaking in Scripture ... repeatedly exhorts believers to lead holy lives. His argument is, You are already Christians; you have already passed from death to everlasting life; you are already saved; therefore, act like Christians. ... (Ephesians 5:8, 15-16). There are dozens of other such exhortations. But our acting like Christians does not save us, for we are already saved. The indicative -- salvation -- logically and chronically precedes the imperative: Behave as the saved people you are. Our good works are not the condition or ground or instrument of our salvation; our salvation is the condition or ground of our obedience. Works and justification are not related as equals, but as effects to cause. Ideas have consequences. Christian ideas have Christian consequences. Those who believe the Gospel, given the opportunity, will produce good works."

(John Robbins, Not Reformed at All: Medievalism in "Reformed" Churches, pp. 121-122)

John Owen: Christ's work of mediation is for His people, not for "all" and "everyone" who ever lived

It was his “church” which he “redeemed with his own blood,” Acts xx. 28; his “church” that “he loved and gave himself for it, that he might sanctify and cleanse it with the washing of water by the word, that he might present it to himself a glorious church,” Eph. v. 25–27. They were his “sheep” he “laid down his life for,” John x. 15; and “appeareth in heaven for us,” Heb. ix. 24. Not one word of mediating for any other in the Scripture. Look upon his incarnation. It was “because the children were partakers of flesh and blood,” chap. ii. 14; not because all the world were so. Look upon his oblation: “For their sakes,” saith he, (“those whom thou hast given me,”) “do I sanctify myself,” John xvii. 19; that is, to be an oblation, which was the work he had then in hand. Look upon his resurrection: “He was delivered for our offences, and was raised again for our justification,” Rom. iv. 25. Look upon his ascension: “I go,” saith he, “to my Father and your Father, and that to prepare a place for you,” John xiv. 2. Look upon his perpetuated intercession. Is it not to “save to the uttermost them that come unto God by him?” Heb. vii. 25. Not one word of this general mediation for all. Nay, if you will hear himself, he denies in plain terms to mediate for all: “I pray not,” saith he, “for the world, but for them which then hast given me,” John xvii. 9.

... To me nothing is more certain than that to whom Christ is in any sense a Saviour in the work of redemption, he saves them to the uttermost from all their sins of infidelity and disobedience, with the saving of grace here and glory hereafter.

(John Owen, The Death of Death in the Death of Christ)