It [the Covenant of Works] commands without any promise of strength at all to perform. There is no such promise to be found in all the Bible, belonging to that covenant. It shews what is to be done, and with all severity exacts the task; but furnishes not anything whereof it is to be made. So the case of men under that covenant is represented by Israel’s case in Egypt, Exod. v. 18, “God therefore now and work,” said Pharaoh to that people; “for there shall no straw be given you, yet shall ye deliver the tale of bricks.
Under the covenant of grace, duty is required, but strength is promised too, Ezek. xxxvi. 27, “A new heart also will I give you, and a new spirit will I put within you; and I will take away the stony heart out of your flesh, and I will give you an heart of flesh.” And the commands in the hands of the Mediator are turned into promises, as appears from Deut. x. 16, “Circumcise the foreskin of your heart, and be no more stiff-necked.” Compare chap. xxx. 6, “And the Lord thy God will circumcise thine heart, and with all thy soul, that thou mayest live.” Yea, the Mediator’s calls and commands to his people bear a promise of help; Prov. x. 29, “The way of the Lord is strength to the upright.
But there is no such thing in the covenant of works; the work must be performed in the strength that was given; they must trade with the stock that mankind was set up with at first: but that strength is gone, that stock is wasted; howbeit the law can neither make it up again, nor yet abate of its demands...” (p. 132-133)
The holiness of God gave out the holy commandment in the covenant, justice annexed the threatening of death to the breach of it, truth secures the accomplishment of the threatening, and so lays the sinner under justice, without relief. So that there is no parting of them, till the utmost farthing be paid (2 Thess. i. 9. punished with Gr. justice or vengeance, everlasting destruction) by the sinner himself, or a cautioner. (p. 162)
Thomas Boston, A View of the Covenant of Works,