I. Setup: The Crude Objection: Moral Facts Require Theism
A crude apologetic argues that an adequate moral theory is impossible apart from theism. But as many theists have pointed out – and as anyone who has taken an introductory course in ethics knows – this view is subject to two serious criticisms: (i) there are a number of plausible ethical theories that make no appeal to God (e.g., utilitarianism, kantianism, and virtue ethics), and (ii) it’s very difficult to give a plausible account of ethics that also requires the truth of theism. Thus, for these sorts of reasons, many Christian philosophers have abandoned this sort of criticism of non-theistic views.
II. The Revised Objection: Non-Theists Lack Sufficient Motivation for the Moral Life
However, some apologists argue that even if non-theists have no special problem in accounting for the existence of morality, they nonetheless have a problem with motivation for adopting and living the moral life. One natural way to spell out this criticism is as follows. Moral motivation must ultimately be grounded in our most basic desires and preferences. But our most basic desires and preferences – at least those relevant to moral motivation – reduce to two: the desire to avoid suffering, and the desire to pursue pleasure/enjoyment. Now this motivational base is sufficiently robust for moral motivation if one is a theist. For then one believes that there is a God who is always watching you – even when no one else is --, and who will reward you for doing what is right and punish you for doing what is wrong, in this life and the afterlife. And if so, then the desire to avoid the suffering that comes from God’s (temporal and eternal) punishment and the desire to pursue God’s temporal (and eternal) rewards are adequate motivators for the moral life.
On the other hand, if one is not a theist, then such a sparse motivational base is inadequate for motivating behavior for an adequate moral life. For such motivational factors are only sufficient when you believe other people -- your family, friends, fellow citizens, and (ultimately) those who enforce the law -- are in a position to discover what you do. But there are too many cases where it's reasonable to think you won't get caught for doing what is wrong or rewarded for doing what is right.
Therefore, given the sparse motivational base that humans come equipped with, non-theistic views of the world cannot provide adequate motivation for a healthy moral life. Only theism, with its belief in a god who is everywhere, and who will punish us for the bad and reward us for the good – including the bad and the good that others do not see -- can provide the requisite resources to motivate one to do the right thing in all the relevant circumstances.
III. Reply: A Thicker Motivational Base
Now it's true that the desire to avoid suffering and to pursue enjoyment are, plausibly, constituents of our motivational base. But there is more in the base than that. For example, we come pre-packaged with the moral emotions of empathy, shame, and guilt. We also come pre-packaged with a disposition to feel revulsion and disgust at certain sorts of moral actions. And all of these things serve as components of the fundamental motivational basis for action in normal human beings as well.
But both the theist and the non-theist can account for these: the theist appeals to God, and the non-theist appeals to the selective pressures of evolution. So, for example, cooperation confers reproductive advantage (groups can defend themselves, gather food, build, etc., better than solitary organisms. Look at ant colonies, lion packs, higher primates, etc., for example), and so "springs" and "triggers" conducive to such behavior are selected. Among these are the moral emotions of empathy, shame, and guilt. And of course a desire for relationships with members of our own species is conducive to cooperation, and will therefore be selected for as well. Furthermore, evolutionary pressures give rise to mechanisms to prevent lethal disease and genetic problems that can lead to human extinction. So, for example, it produces the mechanisms of revulsion and disgust that repel us from actions that threaten species survival, such as incest.
Thus, it appears that the fundamental motivational basis for moral action is much more robust than the theist assumes in the revised objection, and this basis can be accounted for in terms of the selective pressures of evolution, without appeal to God. But if that's right, then the revised objection is undercut. For the elements of the base just sketched look to be sufficient to motivate moral behavior even in contexts of the sort highlighted in the objection, viz., when no one is watching us, and the chances of getting caught for wrongdoing are slim. So, for example, the natural desire for relationships and the natural bonds of empathy prevent us from seriously harming one another in normal conditions, as do the pangs of shame and guilt at the thought of doing so. The inner springs of the moral life therefore seem sufficient for moral behavior without appeal to the external aid of an omniscient observer.
IV. First Criticism: It’s Still Not Enough
Now suppose one concedes that the motivational base within human nature is fairly robust, and that a non-theist can account for such a base in terms of evolutionary factors. Nevertheless, someone might object that such motivational factors in our constitution are still an insufficient basis for moral motivation. Thus, someone might argue that the moral emotions are far too weak, qua motivators, to move us to do the right thing when required. By contrast, the fear of eternal punishment and the desire for eternal rewards are sufficiently strong to motivate us to do the right thing, even when doing so goes against our short-term interests -- and even the long-range interests that shape our lives on this earth. Therefore, only the latter two constituents in our motivational base are sufficient for moral motivation.
Is the criticism above a good one? The first thing to note is that this is an empirical claim: it’s not knowable a priori; furthermore, this empirical claim goes beyond common knowledge. We will therefore need some good evidence for this claim, such as statistics about moral behavior in societies that are predominantly non-theistic in their beliefs. And of course there are reports on violence and crime rates in other countries. But as it turns out, crime rates in predominantly secular countries (e.g., Canada and Western Europe) are actually much lower than they are than in predominantly theistic-believing countries like the Unites States (Here are some comparative statistics on murder rates, for example. See also here and here.). So, it appears that the objection under consideration is not supported by the relevant empirical evidence: the constitutional factors I mentioned appear to be sufficient motivators for ethical behavior.
V. Second Criticism: Why Follow Your Basic Inclinations? Tu Quoque
But someone might say, "Why follow those inclinations?" My answer: that's like asking, "Why not eat a shit sandwich?" For, first, in both cases, we seem to have reached explanatory bedrock with our motivational base: there's no further answer beyond, "because I and other normal humans find it repulsive, or at least uninteresting. That's just the way humans are constituted -- such an action goes against the grain of the basic desires, aims, and interests that come with the human package." And second, in both cases, no further answer seems needed: the answer to the shit sandwich question is satisfactory to everybody; if so, then by analogy, so should the moral motivation question, it seems to me.
Beyond this, of course, there is the issue of parity: Why should the theist be moral? Because if they don't they'll be punished? But then why care if you're punished? Why care about your own welfare? Why should those sorts of considerations motivate you? If you appeal to brute factors of human constitution ("well, we're just constituted to be motivated by such factors, without appeal to anything deeper. God has constituted us to see such factors as a fundamental basis for action"), then the non-theist can do the same. On the other hand, if it's not good enough for the non-theist, why think it's good enough for the theist? We can put the point another way: No matter what view one holds, there's going to be an explanatory stopping-point in terms of the bedrock desires and interests that humans are constituted to take as a fundamental basis and motivation for action.
To sum up: many Christian philosophers today grant that the non-theist can provide plausible ethical theories – including theories that entail that morality is objective in the strongest sense – without appeal to God (e.g., utilitarianism, Kantianism, and virtue ethics). However, some apologists grant that even if non-theists can account for the existence of morality, only the theist has an adequate basis for motivating the moral life. But we have seen that there is no good reason to think so. This is because (i) we come pre-packaged with a motivational base that’s sufficiently robust to motivate the moral life, (ii) this motivational base can be accounted for without appeal to God, and (iii) there is empirical evidence that this base is sufficient for motivating the moral life without resorting to belief in God. And if these things are so, then argument from moral motivation appears to be undercut.