The following is adapted from John Newton's Letter 30.

Ignorance of the nature and design of the law is at the bottom of most religious mistakes. This is the root of self-righteousness, the grand reason why the gospel of Christ is no more regarded, and the cause of that uncertainty and inconsistency in many who, though they profess themselves teachers, understand neither what they say nor what they affirm.

If we begin first by stating what is meant by the law, and then by what means we know the law to be good, I think it will be easy to conclude, from these premises, what it is to use the law lawfully.

What is Meant by the Law

The law, in many passages of the Old Testament, signifies the whole revelation of the will of God, as in Psalm 1:2 and 19:7. But the law, in a strict sense, is contradistinguished from the gospel. Thus the apostle considers it at large in his epistles to the Romans and Galatians. I think it evident that the apostle is speaking of the law of Moses in 1 Timothy 1:8. But, to have a clearer view of the subject, it may be proper to look back to a more early period.

The law of God, then, in the largest sense, is that rule or prescribed course which he has appointed for his creatures according to their various natures and capacities, that they may answer the end for which he has created them.

Inanimate Creation

Thus it comprehends the inanimate creation—the wind or storm fulfills his word, or law. He has appointed the moon for its seasons, and the sun knows its going down or going forth, and performs all its revolutions according to its Maker's pleasure. If we could suppose the sun was an intelligent being, and should refuse to shine, or should wander from the station in which God has placed it, it would then be a transgressor of the law. But there is no such disorder in the natural world. The law of God in this sense, or what many choose to call the law of nature, is no other than the impression of God's power, whereby all things continue and act according to his will from the beginning: for "he spake, and it was done; he commanded, and it stood fast" (Ps. 33:9).

Animals

The animals, destitute of reason, are likewise under a law; that is, God has given them instincts according to their several kinds for their support and preservation, to which they invariably conform. A wisdom unspeakably superior to all the contrivances of man disposes their concernments and is visible in the structure of a bird's nest, or the economy of a beehive. But this wisdom is restrained within narrow limits; they act without any remote design and are incapable either of good or evil in a moral sense.

Man

When God created man, he taught him more than the beasts of the earth and made him wiser than the birds of heaven. He formed him for himself, breathed into him an immortal spirit incapable of dissolution, gave him a capacity not to be satisfied with any creature-goodness, endowed him with an understanding, will, and affections, which qualified him for the knowledge and service of his Maker, and a life of communion with him.

The law of God, therefore, concerning man, is that rule of disposition and conduct to which a creature so constituted ought to conform, so that the end of his creation might be answered, and the wisdom of God be manifested in him and by him. Man's continuance in this regular and happy state was not necessary as it is in the creatures who, having no rational faculties, have properly no choice, but act under the immediate agency of Divine power.

The Fall

As man was capable of continuing in the state in which he was created, so he was capable of forsaking it. He did so, and sinned by eating the forbidden fruit. We are not to suppose that this prohibition alone consisted of the entire law to Adam, so that if he had abstained from the tree of knowledge, he might in every other respects have done (as we say) whatever he pleased. Rather, this injunction was but the test of his obedience; and while he regarded it, he could have no desire contrary to holiness, because his nature was holy. But when he broke through the injunction, he broke through the whole law—and stood guilty of idolatry, blasphemy, rebellion and murder. The divine light in his soul was extinguished, and the image of God defaced; he became like Satan, whom he had obeyed, and lost his power to keep the law which was connected with his happiness.

Yet still the law remained in force. The blessed God could not lose his right to that reverence, love and obedience which must always be due to him from his intelligent creatures. Thus Adam became a transgressor and incurred the penalty of death. But God, who is rich in mercy, and according to his eternal purpose, revealed the promise of the Seed of the woman and instituted sacrifices as types of that atonement for sin, which he in the fullness of time should accomplish by the sacrifice of himself.

After his fall, Adam was no longer a public person—though he himself was saved by grace through faith, the depravity he had brought upon human nature remained. His children—and so all his posterity—were born in his sinful likeness, without either ability or inclination to keep the law. The earth was soon filled with violence.

But a few in every successive age were preserved by grace and faith in the promise. Abraham was favored with a more full and distinct revelation of the covenant of grace; he saw the day of Christ and rejoiced (Jn. 8:56). In the time of Moses, God was pleased to set apart a peculiar people to himself, and to them he published his law with great solemnity at Sinai. This law consisted of two distinct parts, very different in their scope and design, though both enjoined by the same authority.

The Ten Commandments

The Decalogue, or Ten Commandments, uttered by the voice of God himself, is an abstract of that original law under which man was created, but published in a prohibitory form. The Israelites, like the rest of mankind, were depraved by sin and strongly inclined to the commission of every evil. This law could not be designed as a covenant, by obedience to which men should be justified, for long before its publication the gospel had been preached to Abraham (Gal. 3:8). But the law entered that sin might abound—that the extent, the evil, and the penalty of sin might be known—for it reaches to the most hidden thoughts of the heart, requires absolute and perpetual obedience, and denounces a curse upon all who continue not therein.

The Ceremonial Law

To the Decalogue was appended the ceremonial or Levitical law, prescribing a variety of institutions, purifications, and sacrifices; the observance of which were, during that dispensation, absolutely necessary to the acceptable worship of God. By obedience to these prescriptions, the people of Israel preserved their legal right to the blessings promised to them as a nation, and which were not confined to spiritual worshipers only. These prescriptions were likewise ordinances and helps to lead those who truly feared God and had conscience of sin to look forward by faith to the great sacrifice, the Lamb of God, who in the fullness of time was to take away sin by the sacrifice of himself. In both these respects, the ceremonial law was abrogated by the death of Christ, and the Jews, then, ceased to be God's peculiar people. Jesus, having expiated sin and brought in everlasting righteousness by his obedience unto death, rendered all other sacrifices unnecessary and vain.

The Gospel and Continuity of the People of God

The gospel supplies that place of the ceremonial law, to the same advantage as the sun abundantly compensates for the twinkling of the stars and the feeble glimmering of moonlight, which are concealed by its glory. Believers of old were relieved from the strictness of the moral law by the sacrifices which pointed to Christ. Believers under the gospel are relieved by a direct application to the blood of the covenant. Both renounce any dependence upon the moral law for justification, and both accept it as a rule of life in the hands of the Mediator, and are enabled to yield it a sincere, though not perfect, obedience.

If an Israelite, trusting in his obedience to the moral law, had ventured to reject the ordinances of the ceremonial, he would have been cut off. In like manner, if any who are called Christians are so well satisfied with their moral duties that they see no necessity of making Christ their only hope, the law, by which they seek life, will be to them a ministration unto death. Christ, and he alone, by faith in his name, delivers us from the curse of the law, having been made a curse for us.

How the Law is Known to be Good

Natural Man Hates the Law

How do we come to know the law to be good? For naturally, we do not and cannot think so (Rom. 8:7). We cannot be at enmity with God and at the same time approve of his law. Rather, this is the ground of our dislike to him: that we perceive the law—by which we are to be judged—as too strict in its precepts and too severe in its threatenings. Men, therefore, so far as in them lies, aim at altering this law. They think it would be better if it required no more than we can perform, if it allowed us more liberty, and especially if it was not armed against transgressors with the penalty of everlasting punishment. This is evident from the usual pleas of unawakened sinners.

Some think: "I am not so bad as some others"—by which they mean God will surely make a difference and take favorable notice of what they suppose good in themselves. Others plead: "If I should not obtain mercy, what will become of the greater part of mankind?"—by which they plainly intimate that it would be hard and unjust in God to punish such multitudes. Others endeavor to extenuate their sins, as Jonathan once said, "I did but taste a little honey, and must I die?"—"These passions are natural to me, and must I die for indulging them?" In short, the spirituality and strictness of the law, its severity, and its leveling effect—confounding all seeming differences in human characters and stopping every mouth without distinction—are three properties of the law which the natural man cannot allow to be good.

Repentance Produced by the Holy Spirit

These prejudices against the law can only be removed by the power of the Holy Spirit. It is his office to enlighten and convince the conscience, and to communicate an impression of the majesty, holiness, justice, and authority of the God with whom we have to do, whereby the evil and penalty of sin is apprehended. The sinner is then stripped of all his vain pretenses, compelled to plead guilty, and must justify his Judge, even though he should condemn him.

It is his office likewise to discover the grace and glory of the Savior as having fulfilled the law for us, and as engaged by promise to enable those who believe in him to honor it with a due obedience in their own persons. Then a change of perception takes place, and the sinner owns the law as holy, just, and good. Then the law is acknowledged to be holy—it manifests the holiness of God, and a conformity to it is the perfection of human nature. There can be no excellence in man except as far as he is influenced by God's law; without it, the greater his natural powers and abilities are, he is all the more detestable and mischievous. It is assented to as just, springing from his indubitable right and authority over his creatures, and suited to their dependence upon him, and the abilities with which he originally endowed them. And though we, by sin, have lost those abilities, his right remains unalienable; and therefore, he can justly punish transgressors.

And as it is just in respect to God, so it is good for man: his obedience to the law and the favor of God therein is his proper happiness—it is impossible for him to be happy in any other way. Only, as I have hinted, to sinners these things must be applied according to the gospel, and to their new relation by faith to the Lord Jesus Christ, who has obeyed the law and made atonement for sin on their behalf; so that through him they are delivered from condemnation, and entitled to all the benefits of his obedience. From him likewise they receive the law as a rule enforced by his own example, and their unspeakable obligations to his redeeming love. This makes obedience pleasing, and the strength they derive from him makes it easy.

What it is to Use the Law Lawfully

We may now proceed to inquire, in the last place, what it is to use the law lawfully. The expression implies that it may be used unlawfully, and it is so by too many.

Abuses of the Law

Legalism

It is not a lawful use of the law to seek justification and acceptance with God by our obedience to it, because it is neither appointed for this end nor capable of answering it in our circumstances. The very attempt is a daring impeachment of the wisdom and goodness of God; for if righteousness could come by the law, then Christ has died in vain (Gal. 2:21; 3:21)! Thus, such a hope is not only groundless but sinful, and no less than willful rejection of the grace of God when persisted in under the light of the gospel.

Antinomianism

Furthermore, it is an unlawful use of the law, an abuse both of law and gospel, to pretend that its accomplishment by Christ releases believers from any obligation to it as a rule. Such an assertion is not only wicked, but absurd and impossible in the highest degree; for the law is founded in the relation between the Creator and the creature, and must unavoidably remain in force so long as that relation subsists. While he is God and we are creatures, in every possible or supposable change of state or circumstances, he must have an unrivaled claim to our reverence, love, trust, service, and submission. No true believer can deliberately admit a thought or a wish of being released from his obligation of obedience to God, in whole or in part—he will rather start from it with abhorrence. But Satan labors to drive unstable souls from one extreme to the other, and has too often succeeded. Wearied with vain endeavors to keep the law that they might obtain life by it, and afterwards taking up with a notion of the gospel devoid of any power, they have at length despised that obedience which is the honor of a Christian, and essentially belongs to his character, and have abused the grace of God to licentiousness. But we have not so learned Christ.

Lawful Uses of the Law

A Means of Conviction of Sin

To speak affirmatively, the law is lawfully used as a means of conviction of sin. For this purpose it was promulgated at Sinai. The law entered that sin might abound: not to make men more wicked—though occasionally and by abuse it has that effect—but to make them sensible how wicked they are.

Having God's law in our hands, we are no longer to form our judgments by the maxims and customs of the world, where evil is called good, and good evil; but rather we are to try every principle, temper, and practice by this standard. Could men be prevailed upon to do this, they would soon listen to the gospel with attention. On some the Spirit of God does thus prevail; then they earnestly make the jailer's inquiry: "What must I do to be saved?" (Acts 16:30). Here the work of grace begins; and the sinner, condemned in his own conscience, is brought to Jesus for life.

A Mirror of God's Majesty

When we use the law as a glass to behold the glory of God, we use it lawfully. His glory is eminently revealed in Christ, but much of it is with a special reference to the law and cannot be otherwise discerned.

We see the perfection and excellence of the law in his life—God was glorified by his obedience as a man. What a perfect character did he exhibit! Yet it is no other than a transcript of the law. Such would have been the character of Adam, and all his race, had the law been duly obeyed. It appears, therefore, a wise and holy institution, fully capable of displaying that perfection of conduct by which man would have answered the end of his creation.

We also see the inviolable strictness of the law in his death. There the glory of God in the law is manifested. Though he was the beloved Son, and had yielded personal obedience in the utmost perfection, yet, when he stood in our place to make atonement for sin, he was not spared. From what he endured in Gethsemane and upon the cross we learn the meaning of that awful sentence, "the soul that sinneth, it shall die" (Eze. 18:4).

A Rule of Life

Another lawful use of the law is to consult it as a rule and pattern by which to regulate our spirit and conversation. The grace of God, received by faith, will dispose us to obedience in general; but, through remaining darkness and ignorance, we are much at a loss as to particulars. We are therefore sent to the law, that we may learn how to walk worthy of God, who has called us to his kingdom and glory; and every precept has its proper place and use.

A Means of Humiliation

Lastly, we use the law lawfully when we improve it as a test whereby to judge of the exercise of grace. Believers differ so much from what they once were, and from what many still are, that without this right use of the law, comparing themselves with their former selves, or with others, they would he prone to think more highly of their attainments than they ought. But when they recur to this standard, they sink into the dust, and adopt the language of Job, "Behold, I am vile; what shall I answer thee? I will lay mine hand upon my mouth. … I cannot answer him one of a thousand" (Job 40:4; 9:3).

Benefits of the Lawful Use of the Law

From hence we may collect, in brief, how the law is good to those who use it lawfully. It furnishes them with a comprehensive and accurate view of the will of God and the path of duty. By the study of the law, they acquire a habitual spiritual taste of what is right or wrong. The exercised believer, like a skillful workman, has a ruler in his hand whereby he can measure and determine with certainty; whereas others judge, as it were, by the eye, and can only make a random guess, in which they are generally mistaken.

Furthermore, by reminding them of their deficiencies and shortcomings, it is a sanctified means of making and keeping them humble; and it exceedingly endears Jesus, the law-fulfiller, to their hearts, and puts them in mind of their obligations to Him, and of their absolute dependence upon him every moment.

The subject is of great importance, and were it rightly understood might conduce to settle some of the angry controversies which have been lately agitated. Clearly to understand the distinction, connection and harmony between the law and the gospel, and their mutual subservience to illustrate and establish each other, is a singular privilege and a happy means of preserving the soul from being entangled by errors on the right hand or the left.