Richard Sibbes (1577–1635) expounds on the differences between the covenant of works and the covenant of grace

Our Rule Is the Covenant of Grace

It will prove a special help to know distinctly the difference between the covenant of works and the covenant of grace, between Moses and Christ.  Moses, without any mercy, breaks all bruised reeds, and quenches all smoking flax.  For the law requires personal, perpetual and perfect obedience from the heart, and that under a most terrible curse, but gives no strength.  It is a severe task master, like Pharaoh's, requiring the whole tale of bricks and yet giving now straw.  Christ comes with blessing after blessing, even upon those whom Moses had cursed, and with healing balm for those wounds which Moses had made.

The same duties are required in both covenants, such as to love the Lord with all our hearts and with all our souls (Deut. 6:5).  In the covenant of works, this must be fulfilled absolutely, but under the covenant of grace it must have an evangelical mitigation.  A sincere endeavor proportionable to grace received is accepted (and so it must be understood of Josiah, and others, when it is said they did that which was right in the sight of the Lord).

(Note: Josiah was the King of Judah who reigned from around 641 BC to 609 BC and whose reign is documented in the Books of 2 Kings and 2 Chronicles.)

Source: Sibbes, Richard (1577-1635), The Bruised Reed, Chapter 6: Marks of the Smoking Flax, 1630.