In the following extract from Ebenezer Erskine’s sermon on Hebrews 10:19-22, the contrast between the covenant of works and covenant of grace as understood by the reformers is put very well:
It is pleasant hence to observe, how the method and order of the covenant of works is just inverted in the covenant of grace. In the covenant of works, duty was the foundation of our privilege ; man was first to perform duty, and upon his doing of that, might expect the privilege in a way of pactional debt. But now, I say, the very reverse of this is God’s order and method in the covenant of grace; for here we are first to believe the privilege, or to receive it as a grant of sovereign grace, and upon that ground we are to go on to duty.
This is a thing that needs to be adverted to with the utmost attention, in regard the very bensil of nature runs in the way of the covenant of works, namely, to expect the privilege on the score of duty, and to fancy that God is a debtor to us, when we have done this and the other duty required in the law whereas the stream of nature runs quite cross to the order and method laid in the covenant of grace, namely, first to receive the privilege in a way of grace, like beggars receiving God’s alms ; and then to perform duty, as a testimony of gratitude for the privilege received, without expecting any thing from the Lord upon the account of duty done by us. This is what proud nature spurns against with the uttermost reluctancy. What? To take all freely, without money and without price,” and to reckon ourselves unprofitable servants when we have done all, is what depraved nature cannot yield to, til’, the heart be new-moulded by sovereign and efficacious grace.
If you have read my previous posts on the covenant in the garden, you’ll know that I believe that the covenantal structure is the same before and after the fall, it wasn’t reversed but perpetuated by a different means. Adam and Eve were to take freely, without duty or merit, money or cost, from the tree of life, and by doing so they’d receive God’s promise of eternal life. Duty, then, would follow grace just as it does with the gospel of Christ and not as the reformers had it in the covenant of works as prior to grace.
The method and order of the covenant were not inverted at the fall, only the means by which the covenant is offered to us, through faith in Christ not the tree of life.