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Most arguments for the existence of God, such as the cosmological argument and teleological argument, are from the ancient world. The ontological argument comes from medieval times. But the moral argument has modern ancestry, emanating from the works of Immanuel Kant.

Kant’s Moral Postulate. Kant strongly rejected traditional arguments for God’s existence (see God, Objections to Proofs for). He did not, however, reject belief in God. Rather, he believed that God’s existence is a practically (morally) necessary postulate, even though we cannot prove it.

Kant’s argument from practical reason for God’s existence, from his Critique of Practical Reason, can be stated:

Happiness is what all human beings desire.
Morality (viz., categorical imperative) is the duty of all human beings (what they ought to do).
The unity of happiness and duty is the greatest good (the summum bonum).
The summum bonum ought to be sought (since it is the greatest good).
But the unity of desire and duty (which is the greatest good) is not possible by finite human beings in limited time.
And the moral necessity of doing something implies the possibility of doing it (ought implies can).
Therefore, it is morally (i.e., practically) necessary to postulate: (a) a Deity to make this unity possible (i.e., a power to bring them together), and (b) immortality to make this unity achievable.

A simpler form goes:

The greatest good of all persons is that they have happiness in harmony with duty.
All persons should strive for the greatest good.
What persons ought to do, they can do.
But persons are not able to realize the greatest good in this life or without God.
Therefore, we must postulate a God and a future life in which the greatest good can be achieved.

Kant never offered his postulate as a theoretical proof for God. He did not believe such proof to be possible. Rather, he viewed God’s existence as a morally necessary presupposition, not the result of a rationally necessary argument.

Kant’s premises are challenged. Existentialists, including Jean-Paul Sartre and Albert Camus and atheists such as Friedrich Nietzsche challenged the assumption that the greatest good is achievable. Although they lived before Kant, Martin Luther and John Calvin, with other Protestant Reformers, denied that ought implies can. Still others, from Aristotle forward, believed the greatest good is achievable in this life.

Rashdall’s Moral Argument. Hastings Rashdall did what Kant never attempted when he offered a rational argument for the existence of God from the moral law. Beginning with the objectivity of the moral law, he reasoned to an absolutely perfect moral Mind (see Hick, 144–52).

An absolutely perfect moral ideal exists (at least psychologically in our minds).
An absolutely perfect moral law can exist only if there is an absolutely perfect moral Mind: (a) Ideas can exist only if there are minds (thoughts depend on thinkers). (b) And absolute ideas depend on an absolute Mind (not on individual [finite] minds like ours).
Hence, it is rationally necessary to postulate an absolute Mind as the basis for the absolutely perfect moral idea.

In support of the objectivity of the absolute moral idea Rashdall offers this reasoning:

Morality is generally understood as objectively binding.
Mature minds understand morality as being objectively binding (i.e., binding on all, not just some).
Moral objectivity is a rationally necessary postulate (because something cannot be judged as better or worse unless there is an objective standard of comparison).
Objective moral ideals are practically necessary to postulate.

If an objective moral law exists independent of individual minds, then it must ultimately come from a Mind that exists independently of finite minds. It is rationally necessary to postulate such a Mind in order to account for the objective existence of this moral law.

The most common ways to challenge this argument are to question the existence of an objective moral law, and to deny that an absolute moral ideal would need an absolute moral Mind. Why cannot a finite mind conjure up the idea of moral perfection without there being any in the real world. After all, cannot we think of perfect triangles without there being one?

Sorley’s Moral Argument. The moral argument is dependent on the objectivity of the moral law. Hence, it is necessary to offer a defense of this premise. This is precisely what W. R. Sorley does in his version of the moral argument for God’s existence. Since there exists a moral ideal prior to, superior to, and independent of all finite minds, there must be a supreme moral Mind from which this moral ideal is derived:

There is an objective moral law that is independent of human consciousness of it and that exists in spite of human lack of conformity to it: (a) Persons are conscious of such a law beyond themselves; (b) Persons admit its validity is prior to their recognition of it; (c) Persons acknowledge its claim on them, even while not yielding to it; (d) no finite mind completely grasps its significance; (e) all finite minds together have not reached complete agreement on its meaning, nor conformity with its ideal.
But ideas exist only in minds.
Therefore, there must be a supreme Mind (beyond all finite minds) in which this objective moral law exists.

Sorley draws attention to an important difference between a natural law and this moral law. The former is descriptive of the universe, while the latter is prescriptive of human behavior. Hence, the moral law cannot be part of the natural world. It is the way humans ought to act. It is beyond the natural world and is the way we should behave in the world.

Critics of Sorley’s form of the moral argument claim that simply because persons believe there is a moral law beyond them and independent of them, does not mean it really is. Following Feuerbach, they believe that such a law is only a projection of human imagination. It is a collective ideal of human consciousness (or unconsciousness), which conjures up the best from human nature as an ideal by which we should live. Critics also point to differences in understanding of morals as an indication that there is no one universal moral law but merely a collection of different human ideals that overlap and are thereby confused as one moral law. Finally, critics challenge the premise that only a supreme, extrahuman Mind can be the basis for this universal moral ideal. Perfect ideas can be created by imperfect minds, they say.

Trueblood’s Moral Argument. Evangelical philosopher Elton Trueblood adds significantly to the moral arguments proposed by Rashdall and Sorley in his form of the argument:

There must be an objective moral law; otherwise: (a) There would not be such great agreement on its meaning. (b) No real moral disagreements would ever have occurred, each person being right from his own moral perspective. (c) No moral judgment would ever have been wrong, each being subjectively right. (d) No ethical question could ever be discussed, there being no objective meaning to any ethical terms. (e) Contradictory views would both be right, since opposites could be equally correct.
This moral law is beyond individual persons and beyond humanity as a whole: (a) It is beyond individual persons, since they often sense a conflict with it. (b) It is beyond humanity as a whole, for they collectively fall short of it and even measure the progress of the whole race by it.
This moral law must come from a moral Legislator because: (a) A law has no meaning unless it comes from a mind; only minds emit meaning. (b) Disloyalty makes no sense unless it is to a person, yet people die in loyalty to what is morally right. (c) Truth is meaningless unless it is a meeting of mind with mind, yet people die for the truth. (d) Hence, discovery of and duty to the moral law make sense only if there is a Mind or Person behind it.
Therefore, there must be a moral, personal Mind behind this moral law.

It is noteworthy that Trueblood’s form of the moral argument argues its validity in terms of its rationality. It reasons, in essence, that to reject the moral law is irrational or meaningless. That is, unless we assume the universe is irrational, there must be an objective moral law and, thereby, an objective Moral Law Giver.

In addition to the things said against the other forms of the moral argument, some critics, especially existentialists and nihilists, simply point to the absurdity of the universe. They simply refuse to assume, with Trueblood, that the universe is rational. They admit that it may be meaningless to assume there is no moral law, but add quickly that this is the way things are—meaningless. Of course, the defender of the moral argument could point to the self-defeating nature of the claim that “Everything is meaningless,” since that very statement is assumed to be meaningful.

Lewis’ Moral Argument. The most popular modern form of the moral argument was given by C. S. Lewis in Mere Christianity. He not only gives the most complete form of the argument in the most persuasive way, but he also answers major objections. The moral argument of Lewis can be summarized:

There must be a universal moral law, or else: (a) Moral disagreements would make no sense, as we all assume they do. (b) All moral criticisms would be meaningless (e.g., “The Nazis were wrong.”). (c) It is unnecessary to keep promises or treaties, as we all assume that it is. (d) We would not make excuses for breaking the moral law, as we all do.
But a universal moral law requires a universal Moral Law Giver, since the Source of it: (a) Gives moral commands (as lawgivers do). (b) Is interested in our behavior (as moral persons are).
Further, this universal Moral Law Giver must be absolutely good: (a) Otherwise all moral effort would be futile in the long run, since we could be sacrificing our lives for what is not ultimately right. (b) The source of all good must be absolutely good, since the standard of all good must be completely good.
Therefore, there must be an absolutely good Moral Law Giver.

The Moral Law Is Not Herd Instinct. Lewis anticipates and persuasively answers major objections to the moral argument. Essentially, his replies are:

What we call the moral law cannot be the result of herd instinct or else the stronger impulse would always win, but it does not. We would always act from instinct rather than selflessly to help someone, as we sometimes do. If the moral law were just herd instinct, then instincts would always be right, but they are not. Even love and patriotism are sometimes wrong.

The Moral Law Is Not Social Convention. Neither can the moral law be mere social convention, because not everything learned through society is based on social convention. For example, math and logic are not. The same basic moral laws can be found in virtually every society, past and present. Further, judgments about social progress would not be possible if society were the basis of the judgments.

The Moral Law Differs from Laws of Nature. The moral law is not to be identified with the laws of nature. Nature’s laws are descriptive (is), not prescriptive (ought) as are moral laws. Factually convenient situations (the way it is) can be morally wrong. Someone who tries to trip me and fails is wrong, but someone who accidentally trips me is not.

The Moral Law Is Not Human Fancy. Neither can the moral law be mere human fancy, because we cannot get rid of it even when we would like to do so. We did not create it; it is impressed on us from without. If it were fancy, then all value judgments would be meaningless, including such statements as “Hate is wrong.” and “Racism is wrong.” But if the moral law is not a description or a merely human prescription, then it must be a moral prescription from a Moral Prescriber beyond us. As Lewis notes, this Moral Law Giver is more like Mind than Nature. He can no more be part of Nature than an architect is identical to the building he designs.

Injustice Does Not Disprove a Moral Law Giver. The main objection to an absolutely perfect Moral Law Giver is the argument from evil or injustice in the world. No serious person can fail to recognize that all the murders, rapes, hatred, and cruelty in the world leave it far short of perfect. But if the world is imperfect, how can there be an absolutely perfect God? Lewis’ answer is simple: The only way the world could possibly be imperfect is if there is an absolutely perfect standard by which it can be judged to be imperfect (see Morality, Absolute Nature of). For injustice makes sense only if there is a standard of justice by which something is known to be unjust. And absolute injustice is possible only if there is an absolute standard of justice. Lewis recalls the thoughts he had as an atheist:

Just how had I got this idea of just and unjust? A man does not call a line crooked unless he has some idea of a straight line. What was I comparing this universe with when I called it unjust.… Of course I could have given up my idea of justice by saying it was nothing but a private idea of my own. But if I did that, then my argument against God collapsed too—for the argument depended on saying that the world was really unjust, not simply that it did not happen to please my private fancies. Thus in the very act of trying to prove that God did not exist—in other words, that the whole of reality was senseless—I found I was forced to assume that one part of reality—namely my idea of justice—was full of sense. [Mere Christianity, 45, 46]

Rather than disproving a morally perfect Being, the evil in the world presupposes a perfect standard. One could raise the question as to whether this Ultimate Law Giver is all powerful but not whether he is all perfect. For if anyone insists there is real imperfection in the world, then there must be a perfect standard by which this is known.

Sources

N. L. Geisler and W. Corduan, Philosophy of Religion
J. Hick, The Existence of God
I. Kant, The Critique of Practical Reason
C. S. Lewis, Mere Christianity
Rashdall, The Theory of Good and Evil
W. R. Sorley, Moral Value and the Idea of God
E. Trueblood, Philosophy of Religion

Norman L. Geisler

Baker Encyclopedia of Christian Apologetics, Baker Reference Library (Grand Rapids, MI: Baker Books, 1999), 498–501.