William Perkins
Works IV, pp. 55-56

The law and gospel are two parts of the Word of God and are divers kinds of doctrine. By the law, I understand that part of God’s Word which promises life to the obeyer. By the gospel, I understand that part which promises it to the believer. These I say are divers kinds of doctrine, to the clearing of which consider: first, their consent and agreement; secondly, their dissent and difference.

First, the law and gospel consent: first in the author, of both which is God; secondly, in their general matter, for both require justice and righteousness to salvation; thirdly, in their end—namely, the glory of God.

Secondly, they dissent in six things:

First, the moral law is written in nature by creation. Yea, and since the Fall we have some remainder of it in us. The Gentiles show the effect of the law written in their hearts (Rom. 2:15). But the gospel is not in nature but above the reach of nature created, much more corrupted. The ground of the law is the image of God, but the ground of the gospel is Jesus Christ.

Secondly, the law will have us do something that we may be saved by it, and that is to fulfill it. The gospel requires no doing of us but only believing in Christ. Objection. But believing is a work to be done. Answer. The gospel requires it not as a work, but as it is an instrument and the hand of the soul to lay hold upon Christ (Rom. 3:21; 4:5; 10:5). Hence is it that the law requires righteousness inherent, but the gospel righteousness imputed.

Thirdly, the law is propounded to the unrepentant sinner to bring him to faith, but the gospel to the believer to the begetting and increase of faith.

Fourthly, the law shows sin, accuses, and reveals justice without mercy; but the gospel covers sin and is a qualification of the rigor of the law. The law says, “Cursed is every one,” etc. (Deut. 27:26). The gospel qualifies that and says, “Except he believe and repent, every man is accursed” (Luke 13:3). Thus the law, which only manifests justice, is moderated by the gospel, which mingles mercy and justice together, justice upon Christ, mercy unto us.

Fifthly, the law tells us what good works must be done; the gospel, how they must be done. The former declares the matter of our obedience; the latter directs us in the manner of obeying. The former is pleased with nothing but the deed; the latter signifies that God is pleased to accept the will and unfeigned endeavor for the deed itself.

Sixthly, the law is no worker of grace and salvation, no, not instrumentally; for it is the ministry of death. The gospel that is preached works grace only, though the law may be a hammer to break the heart and prepare the way to faith and repentance.

Adversaries. The papists, who hold that they are one doctrine only, but herein differing: that the law is more dark, the gospel more plain; the former more hard to fulfill, the latter more easy; that is as the root of a tree, this as the body and branches. By which premises they would conclude Christ to be no Savior but an instrument rather for us to save ourselves by His giving us grace to keep the law; for a sinner must needs be saved by works, if there is no difference between the law and the gospel, and if the law which requires works were not moderated by the gospel, which requires not works but faith.