The doctrine of republication of the Covenant of Works at Sinai should not be taken as an indication that somehow Israel was under a Covenant of Works for salvation, though it is sometimes, by various parties, derided as such. That would be a gross misapprehension - and Fisher, and Boston through his footnotes in The Marrow, argue this quite emphatically.
"Nomista: But, sir, were the children of Israel at this time better able to perform the condition of the covenant of works, than either Adam or any of the old patriarchs were, that God renewed it now with them, rather than before?The footnote indicated at the * is an important restatement by Boston, in which he writes,
Evangelista: No, indeed; God did not renew it with them now, and not before, because they were better able to keep it, but because they had more need to be made acquainted what the covenant of works is, than those before... So that you see the Lord's intention therein was, that they, by looking upon this covenant might be put in mind what was their duty of old, when they were in Adam's loins; yea, and what was their duty still, if they would stand to that covenant, and so go the old and natural way to work; yea, and hereby they were also to see what was their present infirmity in not doing their duty: that so they seeing an impossibility of obtaining life by that way of works, first appointed in paradise, they might be humbled, and more heedfully mind the promise made to their father Abraham, and hasten to lay hold on the Messiah, or promised seed.
Nomista: Then, sir, it seems that the Lord did not renew the covenant of works with them, to the intent that they should obtain eternal life by their yielding obedience to it?
Evangelista: No, indeed; God never made the covenant of works with any man since the fall, either with expectation that he should fulfil it, or to give him life by it; for God never appoints any thing to an end, to the which it is utterly unsuitable and improper. Now the law, as it is the covenant of works, is become weak and unprofitable to the purpose of salvation; and, therefore, God never appointed it to man, since the fall, to that end. And besides, it is manifest that the purpose of God, in the covenant made with Abraham, was to give life and salvation by grace and promise; and, therefore, his purpose in renewing the covenant of works, was not, neither could be, to give life and salvation by working; for then there would have been contradictions in the covenants, and instability in him that made them. Wherefore let no man imagine that God published the covenant of works on Mount Sinai, as though he had been mutable, and so changed his determination in that covenant made with Abraham; neither, yet let any man suppose, that God now in process of time had found out a better way for man's salvation than he knew before: for, as the covenant of grace made with Abraham had been needless, if the covenant of works made with Adam would have given him and his believing seed life; so, after the covenant of grace was once made, it was needless to renew the covenant of works, to the end that righteousness of life should be had by the observation of it. The which will yet more evidently appear, if we consider, that the apostle, speaking of the covenant of works as it was given on Mount Sinai, says, "It was added because of transgressions," (Gal 3:19). It was not set up as a solid rule of righteousness, as it was given to Adam in paradise, but was added or put to;* it was not set up as a thing in gross by itself." (pp. 61-63, The Marrow of Modern Divinity)
"It was not set up by itself as an entire rule of righteousness, to which alone they were to look who desired righteousness and salvation, as it was in the case of upright Adam, "For no man, since the fall, can attain to righteousness and life by the moral law," Lar. Cat. quest. 94. But it was added to the covenant of grace, that by looking at it men might see what kind of righteousness it is by which they can be justified in the sight of God; and that by means thereof, finding themselves destitute of that righteousness, they might be moved to embrace the covenant of grace, in which that righteousness is held forth to be received by faith. (p. 63, footnote, The Marrow of Modern Divinity)In no sense was the republication (or re-presentation) of the Covenant of Works at Sinai taken to be a replacement of the Abrahamic promise - the Covenant of Grace, whereby salvation was taught to us as being by grace through faith in Christ. The Sinai presentation of the Covenant of Works, rather, has much in common with Calvin's first use of the Law - for a good post on this topic, see Creed or Chaos here, and here. There is MUCH more to be said on the doctrine of republication as taught in the Marrow... but alas, it will have to wait til another time.