Obj. 10. This makes Christ to have done that very thing (for matter) which we ourselves should, that he paid that very debt of obedience in kind (and not in value only) which the law required, and which we should have paid; which if so, and that that be reckoned to us, we are then justified by works, our righteousness is legal rather than evangelical.
Ans. I have had occasion, in what went before, to speak a little of the idem and tantundem, as they refer to Christ’s sufferings, in answer to that question, “Whether he suffered the self-same penalty which threatened and the sinner himself should have endured? or whether he suffered only that which was equivalent thereunto?” In the deciding of which I closed with the common determination, that Christ’s sufferings, for kind and substance, were the same which the law threatened; but as to some certain circumstances and accidents they were but equivalent. The same resolution I shall give concerning the idem and tantundem with respect to his active obedience. As to the substantial duties required by the moral law, and them in kind he submitted, and to that very obedience which we were obliged unto; so it was the idem. By then there were some circumstances (arising from some special considerations about his person) which in other things made a difference; with respect to which it was but the tantundem. What all were bound to do in the great and indispensable duties of the law (as holiness, love to God, &c.,) that Christ did; but what some only are bound to do, upon certain special obligations lying upon them as they stand in such and such relations (as magistrates, husbands, &c.,), that was not done by Christ in specie (he not standing in those relationships). In the substantial duties of the law, and in those acts of obedience which were in general necessary, Christ did just that which we should have done; (understand me that I speak of legal, not of evangelical obedience; for though Christ did that for us which the law demanded, yet he did not do that for us which the gospel demands.) But as to some particular duties of the law proper to such persons in such circumstances, those he, not being under those circumstances, did not do; and yet there is no defect in his obedience, the want of this particular being supplied and made up by his general obedience. The text says “that the righteousness of the law might be fulfilled in us”; now why may we not content ourselves with this, that Christ fulfilled the law’s righteousness, without running of ourselves upon perplexing debates about the idem and the tantundem? The case (in brief) stands thus: the law must be obeyed, in ourselves we neither did nor could obey it, our surety, therefore, must do it for us. He doing it for us, his obedience must be imputed to us. This imputation must be of that very obedience which we were bound unto; otherwise, (this, and not something else in the lieu of it, being demanded by the law,) we are yet debtors to the law. Therefore it follows that Christ did the idem which we should have done. For as he delivered us from the curse of the law by bearing that very curse in his own person which we should have borne, so he fulfilled the righteousness of the law for us by conforming to that very righteousness in his own person which we should have come up to.
Thomas Jacomb, Several Sermons Preach’d on the whole Eighth Chapter of the Epistle to the Romans (London: Printed by W. Godbid, and are to be Sold by M. Pitt at the White-Hart in Little-Britain, and R. Chiswel at the Rose and Crown, and J. Robinson at the Golden Lion in St. Pauls Church yard, 1672), 608-609. [Some spelling modernized, and italics original]