If objective moral truths exist, God must exist.
Premise one addresses the question how are moral truths grounded in reality? The MA argues that the key fact of reality that explains the existence of moral truths is God. But, is this right? As with the first premise, a few alternative explanations are offered. Among them are 1. Moral Naturalism; 2. Individual Subjectivism; 3. Cultural Subjectivism; 4. Positive Law; 5. Utilitarianism; 6. Brute Fact/Platonism. After examining these possibilities, we will conclude that the best explanation for the existence of objective moral truths is God.
Moral Naturalism is the moral aspect of evolutionary naturalism, the prevailing theory of science for explaining the origin of human life. It argues that man evolved moral instincts as a survival adaptation. A hypothetical example might be something like food sharing. The theory holds that if everyone agrees to share food and other valuable resources (to act morally that is), then more members of the species will survive and pass on their DNA to offspring. Since this behavior carries an adaptive advantage, it is argued that it was adopted by our ancient forbears as “moral.” Describing this theory's conclusion about the origin of morals, Michael Ruse writes, "Morality then is not something handed down to Moses on Mount Sinai. It is something forged in the struggle for existence and reproduction, something fashioned by natural selection.” In other words, according to evolutionary naturalism, one should not murder not because it is absolutely wrong, but because killing one’s fellow man does not serve the highest good of the species. There are, however, two significant problems with this naturalistic explanation of moral truths which may be called: 1. The Herd-Morality Problem; and 2. The Value Problem.
The Herd-Morality Problem: Craig puts it this way:
On a naturalistic view moral values are just the behavioral byproducts of biological evolution and social conditioning. Just as a troop of baboons exhibits cooperative and even self-sacrificial behavior because natural selection has determined it to be advantageous in the struggle for survival, so their primate cousins homo sapiens have evolved a sort of herd morality for precisely the same reasons. As a result of socio-biological pressures, there has evolved among homo sapiens a sort of herd morality which functions well in the perpetuation of our species. But on the atheistic view, there doesn't seem to be anything that makes this morality objectively binding and valid.
Craig's point is that if Evolution is true, then "moral truths" aren't true. They are more like habits of behavior that we have adopted over time for the sake of survival. Consequently, they could have been different if the process of evolution itself had gone differently. Interestingly, Darwin anticipated this problem writing:
If … men were reared under precisely the same conditions as hive-bees, there can hardly be a shadow of doubt that our unmarried females would, like the worker-bees, think it a sacred duty to kill their brothers, and mothers would strive to kill their fertile daughters, and no one would think of interfering.
Richard Dawkins draws out the true meaning of this fact, saying,
There is at bottom no design, no purpose, no evil, no good, nothing but pointless indifference. . . We are machines for propagating DNA. . . It is every living object’s sole reason for being.
Notice, the inescapable implication of a herd-morality is that there is no evil and no good. Morals are not absolute statements about right and wrong. Instead, they are only beneficial rules for the preservation of a species. Michael Ruse also points this out saying,
Morality is a biological adaptation no less than are hands and feet and teeth. …Considered as a rationally justifiable set of claims about an objective something, ethics is illusory. I appreciate that when somebody says ‘Love thy neighbor as thyself,' they think they are referring above and beyond themselves. … Nevertheless, … Such reference is truly without foundation. Morality is just an aid to survival and reproduction, … any deeper meaning is illusory.
In other words, on evolutionary naturalism, there are no objective moral truths (see Moral Nihilism above). Moral naturalism does not explain the existence of objective moral truths. Rather, it explains them away.But, again, the great majority of people agree that objective morals exist and are more than an illusion—they are objective truths of reality which could never be different. For this reason, we must reject evolutionary naturalism as a viable explanation for the existence of objective moral truths.
 Ruse, Michael. “God Is Dead. Long Live Morality | Michael Ruse.” The Guardian, Guardian News and Media, 15 Mar. 2010, www.theguardian.com/commentisfree/belief/2010/mar/15/morality-evolution-philosophy.
 Craig, William Lane. “Navigating Sam Harris' The Moral Landscape.” Reasonable Faith, 1 Apr. 2018, www.reasonablefaith.org/writings/popular-writings/existence-nature-of-god/navigating-sam-harris-the-moral-landscape/.
 Darwin, Charles. The Descent of Man. Shine Classics, 2014.
 Dawkins, Unweaving the Rainbow (London: Allen Lane, 1998), cited in Lewis Wolpert, Six Impossible Things before Breakfast (London: Faber and Faber, 2006), p. 215.
 Ruse, Michael. “Evolutionary Theory and Christian Ethics: Are They in Harmony?” Zygon, vol. 29, no. 1 (March 1994), 5-24.