Here's a great quote from Charles Haddon Spurgeon:
"The doctrine of the covenant lies at the root of all true theology. It has been said that he who well understands the distinction between the covenant of works and the covenant of grace, is a master of divinity.
I am persuaded that most of the mistakes which men make concerning the doctrines of Scripture, are based upon fundamental errors with regard to the covenant of law and of grace. May God grant us...the power to instruct, and...to receive instruction on this vital subject."