What is the gospel itself but a merciful moderation, in which Christ's obedience is esteemed ours, and our sins laid upon him, wherein God, from being a judge, becomes our Father, pardoning our sins and accepting our obedience, though feeble and blemished? We are now brought to heaven under the covenant of grace by a way of love and mercy.
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 taskmaster, like Pharaoh's, requiring the whole tale of bricks and yet giving no 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).
The law is sweetened by the gospel, and becomes delightful to the inner man (Rom:7:22). Under this gracious covenant, sincerity is perfection. This is the death in the pot of the Roman religion, that they confound the two covenants, and it deadens the comfort of drooping ones that they cannot distinguish them. And thus they suffer themselves to be held under bondage when Christ has set them free, and stay in the prison when Christ has set open the doors before them.
--Richard Sibbes, "The Bruised Reed," pgs. 36-37