What is the earliest claim that Jesus Christ kept the law of Moses on behalf of others? J N Darby and William Kelly, those responsible for the Plymouth Brethren movement in the mid to late 1800s, did not accept the concept that, during his earthly life, Jesus Christ kept the law on behalf of believers. Referring to the ‘righteousness of God’ (Note: not that of Jesus Christ) JND states :

It is His own (God’s) righteousness which is upon the believer (1)

I can find, myself, nothing in the early Church Fathers, and nothing in the writings of the Reformers on the subject.
John Calvin in his Institutes (1560) writes :

… since, as God only, he (Christ) could not suffer, and, as man only, could not overcome death, he united the human nature with the divine, that he might subject the weakness of the one to death as an expiation of sin, (2)

The Helvetic Confession (1564) states :

Therefore, because faith receives Christ our righteousness and attributes everything to the grace of God in Christ, on that account justification is attributed to faith, (3)


…  but the righteousness of Christ, or rather, he imputes faith in Christ to us for righteousness. (3)

Although the Helvetic Confession refers to ‘the righteousness of Christ’ (a collocation never found in scripture) the phrase is qualified by ‘or rather’ and it is made clear that ‘faith in Christ’ is in view, not the idea that Christ kept the law on behalf of anyone.
In 1647 the Westminster Confession contains the wording :

Those whom God effectually calleth, He also freely justifieth:a not by infusing righteousness into them, but by pardoning their sins, and by accounting and accepting their persons as righteous, not for anything wrought in them, or done by them, but for Christ’s sake alone; nor by imputing faith itself, the act of believing, or any other evangelical obedience to them, as their righteousness, but by imputing the obedience and satisfaction of Christ unto them. (4)

Here the words ‘imputing the obedience … of Christ’ occur but without a definition of what that actually means. In context, one would expect it to mean what Calvin means, that is to say, Christ’s obedience to the Father in offering himself up to sufferings and death.
It is only when we come to the modified form of the Westminster Confession - the Savoy Declaration of 1658 that we have the words :

by imputing Christ's active obedience to the whole law, and passive obedience in his death for their whole and sole righteousness, they receiving and resting on him and his righteousness by faith (5)

This definitive wording (active and passive obedience) is most probably the influence of John Owen who was involved in the preparation of the Savoy Declaration and who, himself, states :

… the obedience of Christ unto the law, and the imputation thereof unto us, are no less necessary unto our justification before God, than his suffering of the penalty of the law, and the imputation thereof unto us, unto the same end. (6)

Is it so that, until 1658, the Christian Church did not teach this doctrine in such a stated form ?
Does such a concept appear in any of the writings of the pre-Nicene or post-Nicene fathers ?
Does such a concept appear in the writings of the Reformers ?
Is this, solely, a ‘Puritan’ doctrine ?

(1) J N Darby Outline of the Epistle to the Romans :  Chapter (2-4) : Para 40
(2) John Calvin Institutes 2.12.3 1560 (Definitive Latin edition)
(3) Helvetic Confession  Chapter 15 on Justification
(4) Westminster Confession  Chapter 11 on Justification
(5) Savoy Declaration Chapter 11 on Justification
(6) John Owen (from The Works of John Owen, vol. V.) Quoted from The Doctrine of Justification by Faith though the Imputation of the Righteousness of Christ; Explained, Confirmed, and Vindicated (1677),
This is the earliest extra-biblical clear statement I have found so far.

Christ therefore ransomed from the curse of the law those who being subject to it, had been unable to keep its enactments. And in what way did He ransom them? By fulfilling it. And to put it in another way: in order that He might expiate the guilt of Adam’s transgression, He showed Himself obedient and submissive in every respect to God the Father in our stead… - St. Cyril of Alexandria (378 – 444 AD) – Commentary on Luke

1 Peter 3:18 &quot;For Christ also suffered once for sins, the righteous for the unrighteous, that he might bring us to God, being put to death in the flesh but made alive in the spirit,&quot; is a more indirect declaration wherein one must make the leap that he was righteous by keeping the Law.