It’s often asserted, or assumed, that God is “necessary” for morality, not infrequently leading to furious argument about the relative virtues of believers and atheists. But there are several senses in which God might be related to morality, so it’s important to distinguish them:

God is necessary for us to know the difference between right and wrong. This is the view that moral truth is a subspecies of revealed truth, i.e., we can only know the difference between right and wrong by consulting the Bible, the church, or some other religious authority. Even within Christianity, though, this is a minority position. The more common view is that we can know the difference between right and wrong using our natural reason and observing the world around us, particularly what actions are conducive to human well-being, etc. There has been a tendency in some self-consciously postmodern theology to see morality as tradition-constituted or embedded in the practices of a particular community, which does in some ways hark back to the morality-as-revealed-knowledge view.

God is necessary for us to be motivated to do right. This may be more common than the previous view. In its crassest form it asserts that the threat of punishment and/or promise of reward by God (whether here or hereafter) is what provides the necessary incentive for us to behave morally. In a more sophisticated vein, one might argue that God’s valuation of the world and the creatures in it provides a motive–if we love God–to treat God’s creatures with respect and care. Yet it seems that atheists, agonostics, and other non-theists can be motivated to moral behavior by many other considerations that make no reference to God. Though one might wonder if positing a transcendent dimension to morality gives an extra incentive to, for instance, more self-sacrificial actions than some alternative views (e.g., naturalism).

God is necessary to make sense of morality. This is the view that God is necessary to account for the deep, metaphysical basis or structure of morality. We might say that God is the “truth-maker” for moral statements. This doesn’t necessarily imply that we must believe in God’s existence to apprehend moral truth; it simply means that what ultimately “makes sense” of morality is God’s existence and/or nature. Most Christians probably hold some version of this view. On the other hand, we may doubt its practical import since it seems possible to have moral beliefs that guide one’s actions without enquiring very deeply into their metaphysical foundation (if any). Not to mention, there are several competing accounts of the metaphsyics of morality, none of which, it’s safe to say, commands anything close to universal assent. And yet it doesn’t seem as though having such a satisfying metaphysical account is necessary for moral discourse to funciton effectively.

I think the second and third views are more plausible than the first, but both would be very difficult to prove. I think one could make a plausible argument that the existence of God makes better sense of moral values than at least some competing views and that belief in God may provide motives for moral virtue that aren’t available to other views, but that falls short of showing that God is necessary for morality in the strong sense sometimes claimed.

ADDENDUM: I left out yet another sense in which it might be said that God is necessary for morality, which seems obvious now that I think of it: God as causally necessary for morally good actions. Most Christians have held that, in some way and to some degree or another, God’s grace empowers us to do good works. This goes beyond merely saying that our knowledge of God’s commands or love acts as a motive for doing good; it’s the claim that God somehow strengthens our wills so that we can do things that, left to our own devices, we would be unable to do. Without this empowering grace, it is sometimes held, we would be utterly incapable of doing good. This also seems to imply that non-believers, to the extent that they do good, are also beneficiaries of God’s grace. Clearly this opens up a whole can of worms concerning God’s providential action in the world, as well as the relationship between divine action and human free will.