Quotes from Dr. Clark’s site:
It may be remarked that the law of the ten commandments was promulgated to Israel from Sinai in the form of a covenant of works. Not that it was the design of God to renew a covenant of works with Israel or to put them upon seeking life by their own obedience to the law but the law was published to them as a covenant of works to show them that without a perfect righteousness answering to all the demands of the law they could not be justified before God and that finding themselves wholly destitute of that righteousness they might be excited to take hold of the covenant of grace in which a perfect righteousness for their justification is graciously provided.
The Sinai transaction was a mixed dispensation. In it the covenant of grace was published as appears from these words in the preface standing before the commandments “I am the Lord thy God which have brought thee out of the land of Egypt out of the house of bondage” and from the promulgation of the ceremonial law at the sametime But the moral law as a covenant of works was also displayed to convince the Israelites of their sinfulness and misery to teach them the necessity of an atonement and lead them to embrace by faith the blessed Mediator the Seed promised to Abraham in whom all the families of the earth were to be blessed.
The law therefore was published at Sinai as a covenant of works in subservience to the covenant of grace And the law is still published in subservience to the gospel as a schoolmaster to bring sinners to Christ that they may be justified by faith Gal iii 24
Robert Shaw, The Reformed Faith: An Exposition of the Westminster Confession of Faith
The covenant of works, which may also be called a legal or natural covenant, is founded in nature, which by creation was pure and holy, and in the law of God, which in the first creation was engraven in man’s heart. For after that God had created man after his own image, pure and holy, and had written his law in his mind, he made a covenant with man, wherein he promised him eternal life, under the condition of holy and good works, which should be answerable to the holiness and goodness of their creation, and conformable to his law. And that nature thus beautified with holiness and righteousness and the light of God’s law, is the foundation of the covenant of works, it is very evident; for that it could not well stand with the justice of God to make a covenant under condition of good works and perfect obedience to his law, except he had first created man pure and holy, and had engraven his law in his heart, whence those good works might proceed. For this cause, when he was to repeat that covenant of works to the people of Israel, he first gave the law written in tables of stone; then he made a covenant with his people, saying, “Do these things, and ye shall live.”
Therefore the ground of the covenant of works was not Christ, nor the grace of God in Christ, but the nature of man in the first creation holy and perfect, endued also with the knowledge of the law. For, as touching the covenant of works, there was no mediator in the beginning between God and man, that God should in him, as in and by a mediator, make his covenant with man. And the cause that there was no need of a mediator was this, that albeit there were two parties entering into a covenant, yet there was no such breach or variance betwixt them that they had need of any mediator to make reconciliation between them; for, as for the covenant of works, God made this covenant [note] with man, as one friend doth with another. For in the creation we were God’s friends, and not his enemies. Thus far of the ground of the covenant of works.
Robert Rollock (1555–99), A Treatise of God’s Effectual Calling in Select works of Robert Rollock