From Thomas Boston’s notes on Edward Fisher’s The Marrow of Modern Divinity:
“Payment of a Double Debt
As man lay in ruins, by the fall guilty and unclean, there stood in the way of his salvation, by mercy designed–
1 the justice of God, which could not admit the guilty creature; and,
2 the holiness of God, which could not admit the unclean and unholy creature to communion with him.
Therefore, in the contrivance of his salvation, it was necessary that provision should be made for the satisfaction of God’s justice, by payment of the double debt mentioned above; namely the debt of punishment and the debt of perfect obedience.” (66)
Our Lord Jesus Christ became surety for the elect in the second covenant (Heb. 8:22); and in virtue of that suretyship, whereby he put himself in the room of the principal debtors, he came under the same covenant of works that Adam did; in so far as the fulfilling of that covenant in their stead was the very condition required of him, as the second Adam in the second covenant. ‘God sent forth his Son; made under the law’ (Gal. 4:4-5). Thus Christ put his neck under the yoke of the law as a covenant of works, to redeem them who were under it as such. Hence he is said to be the ‘end of the for righteousness to every one that believeth’ (Rom. 10:4); namely, the end for consummation, or perfect fulfilling of it by his obedience unto death, which presupposeth his coming under it. And thus the law as a covenant of works was magnified and made honourable; and it clearly appears how ‘by faith we establish the law’ (Rom. 3:31). How then is the second covenant a covenant of grace? In respect of Christ, it was most properly and strictly a covenant of works, in that he made a proper, real, and full satisfaction in behalf of the elect; but in respect of them, it is purely a covenant of richest grace, in as much as God accepted the satisfaction from a surety, which he might have demanded of them; provided the surety himself, and gives all t o them freely for his sake.” (66,67).