The Scriptures are clear that God has revealed his law to men through Scripture via the Mosaic Law and in nature through the created order and conscience. It is also clear that these two media are capable of guiding and condemning Christian conduct. But are there alternative ethical philosophies that are both ethically adequate and divinely warranted? This essay shall examine biblically the Christian philosophy of ethics and the procedure for making decisions while also considering the relationship between the two forms of law previously mentioned. Specifically, we shall examine whether the distinction between Mosaic and natural law is simply a difference in terminology in which both loci of law share the same content, or if the distinction is a reflection of two distinct law codes.
Our first task is to survey the major ethical philosophies to show that the Christian only has two legitimate choices. The approach of this survey is to define each method of ethics, then to rhetorically and logically reduce the options according to their viability. We shall conclude by demonstrating that there is but one feasible method, as the two viable options theoretically and theologically distill into one. The purpose of surveying illegitimate methods will be made clear progressively. I will be following Dr. Harold O. J. Brown’s fine categorization of meta-ethics under two main branches: Positivistic and Absolute ethics.
Positivistic ethics are moral rules and codes that are arbitrarily posited by men. This category of ethics finds its source and authority in the human authorities themselves. We shall briefly consider four types under this class.
Relativistic ethics come in two varieties, cultural and individual. Cultural relativism asserts that morality is established on the basis of group consensus. One does not have the right to cheat on his wife, for the community has established that such is unlawful. This option is not available to the Christian, for God strongly judged pagan nations for their self-established ethics, and He did so precisely because in choosing their own they parted from his. One thinks of the philosophical absurdity of a cultural sub-group who, contrary to the larger group whose consensus forbids adultery, forms its own consensus that adultery is permissible. Here would exist two groups with competing, yet, lawful ethics, as both are established by a community. Cultural relativism is variant and as such its laws cannot be said to be just, for justice is invariant by definition.
The individual form of relativism is more radical, as on such a model individuals may now ignore their community norms to do what is right in their own eyes. This hardly needs a biblical refutation, other than to say that it views the individual more ultimate than God, and thus idolatrous. Of course, it suffers the same critique mentioned previously concerning its flawed concept of justice.
Very similar to individual relativism, this form of ethics states that morality is a matter of one’s personal inward “authenticity.” What matters is not an ethical code external the person, but the person’s own character and motive. Outside of what a person considers authentic, there are no rules. Socrates and John Paul Sartre were notable proponents of this view.
This option is not available to believers, because it also ignores God’s revelation that He, not his creation, decides what is right. Faithfulness to one’s wife may not seem authentic at times, but one must still be faithful. One may have a good motive to steal (e.g. to help the poor), but such is unlawful regardless of purpose.
John Stuart Mill and Jeremy Bentham are famous proponents of teleological or utilitarian ethics. This view is very popular today and can be tempting to many Christians. Teleological ethics judges morality on the basis of the results of the choice–simply put, the end justifies the means. Murder is wrong according to this ethic because if a society were to murder this would fail to bring about the greatest amount of happiness to the greatest amount of people.
For the Christian a choice may not be made on the basis of the results of the choice, or pragmatically. The Scriptures warn against being wise in one’s own eyes (Isaiah 5:21). Moreover, is it always wise to do what brings happiness to the greatest number? Perhaps murdering a class of minorities would bring about the better good for the majority whose taxes often support the former. Yet we know that God says universally, “Thou shall not kill.”
Our survey reveals that Christians are still in need of an ethical philosophy that comports with the universal nature of God’s commands.
Sometimes called one-norm ethics, this is the view of Joseph Fletcher. Situational ethics does not hold to a universally valid morality. What is properly moral depends on the situation, specifically a morality that serves love.
But for the Christian love is never outside the context of law. Jesus said “If you love me, obey my commands” (John 14:15). Situational ethics can not condemn a necrophiliac for killing a person for his own devices, for he was motivated by his “love” for sexual perversion.
All positivistic ethics fail because they are arbitrary and subjective. Subjective ethics do not comport with the biblical teaching that man may not do what is right in his own eyes. Moreover, a cursory reading of the way in which commandments are delivered in Scripture reveal a “thus saith the Lord” nature of meta-ethics. Therefore, the standard of morality must be external to man, and universal to all men.
We now consider a category of ethical philosophies that attempts to avoid the subjective and arbitrary nature of positivistic ethics. Absolutist ethics hold that values exist in and of themselves, and they are therefore universal.
We begin with the Deontological model which teaches that ethics have a necessity about them. Plato, David Hume and G.E. Moore were notable deontologists. The system was made most popular by Immanuel Kant who taught that right ethics are those performed for the right reason, at all times and situations, and by all people—thus a categorical imperative. Duty must be done for duty’s sake.
But how can a Christian know which duties are to be performed? Kant says, “Act only according to that maxim whereby you can at the same time will that it should become a universal law.”  But who says? This ethic assumes the right of man to judge which moral choice is proper.
The major flaw with deontological ethics is that they lack content and thus resort to subjectivism. It is one thing to to say that ethics are categorical and must be done for duties sake, and quite another to define what exactly should be done. Ironically, while Kant attempts to escape the subjective nature of ethical philosophies, his method is throughly positivistic in practice and is thus subject to the criticisms of subjectivism above.
Natural law is a moral theory which presupposes that things have a natural goal inherent in them. Common sense is emphasized in natural law, and it is seemingly not without scriptural warrant: the apostle Paul, for example, appeals to it in the first to chapters of Romans. This theory seems to garnish unassailable support, for if God says that men know things about Him and morality by nature, then that suffices for Christians. Our only task now is to consider any other viable biblical theories of ethics and to draw a relationship between them.
The last ethical method we survey is Divine Command ethics which is also called metaphysical moralism, for it holds that law comes from a source completely outside of nature. Christians as notable as Augustine and Calvin have held this view. This view states that the only sufficient ethical guide is the revealed, inscripturated Word of God. The laws divinely given to Moses and written down are our guide.
Relationship of Mosaic and Natural law
Having surveyed the options and found all but the last two theories biblically untenable, we now turn to examine the relationship between Mosaic Law and natural law. It is biblically manifest that God has revealed his law to his covenant people, having delivered them to Moses and written them on tablets. Yet, natural law is available “naturally” to non-covenant and covenant peoples alike. The questions now are: 1) what is the relationship between Mosaic and natural law, and 2) do both serve as valid meta-ethic for Christians?
Distinctions and Similarities
For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves: Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another.
Moreover, it would appear that the Gentiles’ knowledge of God’s law extends beyond the basic apodictic principles of the Ten Commandments. Deuteronomy 4:5-8 indicates that the extradecalogical laws (causuistic or case laws) were not only a code for Israel, but also a model for the surrounding nations who had only natural law. Leviticus 18:24-27 states that the other nations were judged and disposed from their land for failing to keep the detailed law code just delivered to Israel.
Consider Amos 1:3-2:3 which is directed toward the non-covenant, pagan nations of Damascus, Gaza, Tyre, Edom, Ammon, and Moab. This passage condemns them via natural law for violating specific casuistic laws given in Scripture to Israel: destroying life in pregnant women (cf. Exodus 21:22-23); slave trafficking ( cf. Exodus 21:16; Deuteronomy 24:7); and defying a corpse (cf. condemned in Deuteronomy 21:23).
We conclude that while the method of delivery is different, the content between Mosaic and natural law is the same.
Having established that Mosaic and natural naw convey the same information and are, therefore, not conflicting standards of ethics, we now shall consider if both are sufficient guides for ethical decision making.
Ethical Procedure and Natural law Problems
The pattern for all ethical reasoning always begins with a general rule or a statement of fact. Next, is a particular observation about the world with which the ethicist wishes to evaluate. Last, is an inference made using the first two premises as proof for the conclusion of the argument: an ethical evaluation.
The following example shall prove that it is impossible to establish the certainty of the first premise of the argument. If one wishes to discover if aborting a baby is murder, he first needs to establish that murder is wrong. Using the Mosaic Law, the Christian has no difficulty establishing this first value—he need only look up the reference in his Bible. However, using natural law one is hard-pressed to establish this first part of the procedure. A Christian might appeal to his conscience that convicts him that murder is wrong. But the non-Christian might also appeal to his, claiming that he has no such conviction. Moreover, even should the pagan happen to share the same conviction as the Christian, how does either discriminate murderous killing from just-war killing, self-defense killing, or capital punishment? He is left again to his own convictions, and not even all Christians agree on these things left to their own consciences.
Natural law as Positivistic
Perhaps the natural law ethicist, having abandoned making an evaluation by his conscience (individual relativism), now looks at the created order and judges that murder is seemingly condemned by the common order of societies. But this is social relativism which was dismissed earlier.
Perhaps he sees in the natural world that the overwhelming majority have condemned murder as an unauthentic choice. Yet, existential ethics was dismissed as well.
The only option remaining is to look at the results in killing, and to see that such produces much strife and chaos indeed. There seems to be the healthy purpose of the creation of families and societies by refraining from murder, both in people and in animals. Certainly the common tendency patently visible throughout nature to preserve life is an honorable goal. The keen reader should note, however, that this is a teleological reading of creation, which is also not an option for the Christian as discussed previously.
Whether for the unbeliever or the believer conscience is an unreliable source of ethics because not all share the same convictions. That God may use natural law to condemn men does not mean that He has chosen to use it unaided by his Spirit and Word to guide men and nations. We have also seen that resorting to creation to learn ethics lends to a positivistic meta-ethic. As shown earlier, Scripture does seem to indicate that natural law does indeed get through to man.
Why, then, is natural law unreliable as a source of ethical evaluation for men?
The key to this question lies in the fallen nature of man who resists what he knows to be true. The Pauline passages mentioned in support of natural law are used by Paul in a condemnatory fashion, not in one that is commendable. In the first chapter of Romans, Paul says that what is known by men naturally is also suppressed. In the second chapter his commendation of the Gentiles for doing what they know is right apart from the (written) law only serves to condemn the Jews, for they have the written law yet failed to obey it.
On this point, Calvin’s insights are helpful. In his immortal Institutes of the Christian Religion, he offers his view on the relationship between the law written on the hearts of men (natural) and the law written down by God in the Scriptures (Mosiac). Calvin says that the”[inward law] engraved on the hearts of all, in a sense asserts the very same things that are to be learned from the two Tables [Ten Commandments].” But due to sinful man’s arrogance and dullness Calvin writes, “the Lord has provided us with a written law to give us a clearer witness of what was too obscure in the natural law, shake off our listlessness, and strike more vigorously our mind and memory.”
We should note that Calvin did not see natural law as insignificant. Though he was clear that it should not take the place of God’s Word, he did see it as useful. First, as a negative aspect it condemns man, rendering him guilty before God, a point we noted earlier. However, it is through it that man knows what is unlawful so that his wickedness is without excuse (Rom. 1:20). Secondly, it is useful in constructing civil government.  As a positive aspect natural law can assist in the prevention of total civil decay in a society. This is not to say that a society can rule rightly without God’s revealed ordinances, but that whatever outward good is achieved apart from God’s word is due to the God-bestowed natural gifts that remain in men. Calvin’s view of this difference, then, is one of lucidity: natural revelation is a sin-obscured edition of special revelation.
A Final Objection
One stock objection given by natural law supporters comes in two forms. The first is that natural law is given to unbelievers, whereas Mosaic Law has been given to believers. Norman Geisler argues for this basic position:
Divine law, therefore, is not given to unbelievers but to believers. Natural law is for unbelievers. Divine law is binding on the church, but natural law is binding on all society. Natural law is directed toward temporal good, but divine law is directed toward eternal good. Inasmuch as natural law reflects the very character of God, it cannot change. Divine law, however, is based on God’s will and therefore does change….This is reflected in God’s change in divine law from the Old Testament to the New Testament. The natural law remains the same from age to age and from person to person.
After seeing the obvious problems with denying the textually verified law of God, hopefully one can see that the task of codifying natural law in Geisler’s terms is difficult. Here again regarding the expressions and applications of natural law we have the same problem of arbitrariness and subjectivity as above. If natural law is not informed by God’s written law then man becomes autonomous in his endeavors to define what he observes in nature.
The second version of this objection is that natural law must be used in preference to the Mosaic Law because the latter was delivered to a redemptive community, and the latter has been given to all men for the work of building cultures. The Mosaic Law, therefore, cannot be used with unbelievers. David Van Drunen has argued:
God has not given covenantally the revelation of Scripture to the world at large. The “indicatives” in Scripture—the things which tell us who we are—are not true of the world at large. And if the “indicatives” do not apply to the unbelievers, the “imperatives” cannot be taken from Scripture and placed upon them. …Generally speaking, at least, argument and discussions with our unbelieving neighbors ought to occur in terms of the natural moral law, not in terms of Scripture—although, of course, Scripture gives us as believers important information and instructions about our lives in various realms. 
Roger Wagner soundly retorts, “The presence or absence of God’s redemptive purpose with respect to an individual or group of human beings does not effect [sic] their need to be directed by the law of God in its verbally revealed expression.” In other words, Van Drunen’s point is arbitrary. But assuming Van Drunen is correct, a legitimate question arises: In such a culture without recourse to a publicly assessable and objective law, to which law does one appeal when disputes arise over conflicting readings of natural law?
To disagree with Calvin that man’s natural capacity to understand is “transitory” and in need of “a solid foundation of truth,” is to affirm the sufficiency of natural law to guide a man apart from Scripture. Moreover, any natural law theory which allows the definition of its laws to run unrestrained by God’s objective word is riddled with flaws: arbitrariness cannot be avoided, and a flight into subjectivism is inevitable. Such a rigid natural/scriptural distinction grants men autonomy to legislate law, and such lawlessness (James 4:11) cannot be permitted in the Christian worldview.
While the Christian is faced with many ethical theories, Scripture only presents him with God’s law as the source of ethics. This law comes through the modes of written revelation and nature (creation and conscience). However, because man is fallen and is bent on suppressing that natural law he must be careful not to use natural law autonomously, and thus out of accord with Scripture. Because the law that is communicated to man in his heart and through the created realm is one and the same with the Mosaic Law, the Christian must turn to the inscripturated law for a sure and reliable ethic.
 Mosaic: Deut 5; Ex. 20; Deut 29:29. Natural: Rom. 1:19:20; 2:14-15.
 Harold O.J. Brown, Unpublished Syllabus for Pastoral and Social Ethics, Reformed Theological Seminary.
 Deut 4:5-8; Lev. 18:24-27
 It is granted that capital punishment, self defense, and just war are the three exceptions to this commandment. The 10 Commandments are summary (apodictic) laws which exemplify and are qualified by the case laws (casuistic) of the Scriptures.
 Should one object that lying is sometimes righteous in the Bible as dictated by the circumstance, two replies are in order. First, not all agree that proper exegesis reveals that lies are commended. Second, if God’s word circumscribes permissible lying, then such is Divine Command ethics and the exceptions would not be considered “lying” proper. See J. Douma, The Ten Commandments, (Phillipsburg: P&R, 1992),pp. 327-328
 See also Prov 3:5-6
 Immanuel Kant, Grounding for the Metaphysics of Morals. 3rd ed, Hackett. Translated by James W. Ellington  (1993). pp. 30.
 Consider Psalm 110 in conjunction with Matthew 5: 177 ff.
 Rom. 1:32
 Rom. 1:20
 Rom. 2:14-15
 John Calvin, Institutes of the Christian Religion, ed. John T. McNeill, trans. Ford Lewis Battles, The Library of Christian Classics, vol. 20 (Louisville: Westminster John Knox Press, 1993), 2.8.2; 368; see also 1.6.1
 Ibid., 2.2.22; 282
 Ibid., 2.2.13; 271-273
 Greg L. Bahnsen, Theonomy in Christian Ethics, Third Edition (Nacogdoches, Texas: CMP, 2002), 387; see Calvin Institutes, 1.6.1
 Norman L. Geisler, Baker Encyclopedia of Christian Apologetics, (Grand Rapids: Baker Books, 1999), 415
 Roger Wagner, Penpoint, Christ Transforming Culture: An Analysis of the Sandlin-Van Drunen “Dialogue” on the Relevance of God’s Law (vol 15, number 8, Dec 2004). See also Van Drunen’s A Case for Natural Law, (Acton Institute, 2006).