The Commentary of Dr. Zacharias Ursinus on the Heidelberg Catechism
4. Christ is the substance and ground of the entire Scriptures. But the doctrine contained in the law and gospel is necessary to lead us to a knowledge of Christ and his benefits: for the law is our schoolmaster, to bring us to Christ, constraining us to fly to him, and showing us what that righteousness is, which he has wrought out, and now offers unto us. But the gospel, professedly, treats of the person, office, and benefits of Christ. Therefore we have, in the law and gospel, the whole of the Scriptures, comprehending the doctrine revealed from heaven for our salvation.
The principal differences between these two parts of the doctrine of the church, consists in these three things:
1. In the subject, or general character of the doctrine, peculiar to each. The law prescribes and enjoins what is to be done, and forbids what ought to be avoided: whilst the gospel announces the free remission of sin, through and for the sake of Christ.
2. In the manner of the revelation peculiar to each. The law is known from nature; the gospel is divinely revealed.
3. In the promises which they make to man. The law promises life upon the condition of perfect obedience; the gospel, on the condition of faith in Christ and the commencement of new obedience. Hereafter, however, more will be said upon this subject in the proper place.