I.     What is the Doctrine of the Church?

The doctrine of the church is the entire and uncorrupted doctrine of the law and gospel concerning the true God, together with his will, works, and worship; divinely revealed, and comprehended in the writings of the prophets and apostles, and confirmed by many miracles and divine testimonies; through which the Holy Spirit works effectually in the hearts of the elect, and gathers from the whole human race an everlasting church, in which God is glorified, both in this, and in the life to come.

This doctrine is the chief and most expressive mark of the true church, which God designs to be visible in the world, and to be separated from the rest of mankind, according to these declarations of scripture: “Keep yourselves from idols.” “Come out from among them, and be ye separate.” “If there come any unto you and bring not this doctrine, receive him not into your houses, neither bid him God speed.” “Be ye holy, touch no unclean thing, ye that bear the vessels of the Lord. Come out of her, my people, that ye be not partakers of her sins, and that ye receive not of her plagues.” (1 John 5:21. 2 Cor. 6:17. 2 John 10. Isa. 52:11. Rev. 18:4.)

II.     What Are The Parts Of The Doctrine Of The Church, And In What Do They Differ From Each Other?

The doctrine of the church consists of two parts: the Law, and the Gospel; in which we have comprehended the sum and substance of the sacred Scriptures. The law is called the Decalogue, and the gospel is the doctrine concerning Christ the mediator, and the free remission of sins, through faith. This division of the doctrine of the church is established by these plain and forcible arguments.

1. The whole doctrine comprised in the sacred writings, is either concerning the nature of God, his will, his works, or sin, which is the proper work of men and devils. But all these subjects are fully set forth and taught, either in the law, or in the gospel, or in both. Therefore, the law and gospel are the chief and general divisions of the holy scriptures, and comprise the entire doctrine comprehended therein.

2. Christ himself makes this division of the doctrine which he will have preached in his name, when he says, “Thus it is written, and thus it behoved Christ to suffer, and to rise from the dead the third day; and that repentance and remission of sins should be preached in his name.” (Luke 24:46, 47.) But this embraces the entire substance of the law and gospel.

3. The writings of the prophets and apostles, comprise the old and new Testament, or covenant between God and man. It is, therefore, necessary that the principal parts of the covenant should be contained and explained in these writings, and that they should declare what God promises and grants unto us, viz: his favor, remission of sins, righteousness, and eternal life; and also what he, in return, requires from us: which is faith and obedience. These, now, are the things which are taught in the law and gospel.

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, consist 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.

Commentary on the Heidelberg Catechism, 1-3.