The simplest form of the moral argument is as follows:
This is a deductively valid argument, which is to say if its premises are true its conclusion cannot be false. The key question is whether or not the premises are true.
The first premise is by far the most often-disputed premise in the argument. While many religious believers take the first premise for granted, the reasons for thinking it true are not clear, and there are some serious objections to it.
Arguments for the first premise
Famous atheists rejected morality
Though this line of argumentation is popular among religious apologists, it clearly commits the fallacy of appealing to authority, and that is not its only problem. Many nontheists who have rejected conventional views of morality have done so on grounds independent of their views on the existence of God. Also, it requires selectively quoting authorities, because many nontheists--indeed, many theists--have rejected the first premise of the moral argument. Finally, it is trivially easy to construct a similar argument against theism, for example: "John Calvin did not believe in free will, therefore 'If God exists, free will does not exist,' but free will does exist, therefore God does not exist." Even if Calvin had good arguments for his stance on free will being entailed by theism, non-Calvinistic theists will not be swayed by the mere citation of Calvin's authority, nor should they be.
Hitler and Stalin were atheists
Main article: 20th century atrocities
The basic fallacy of this argument is similar to the one in the first, though it could be considered an example of guilt by association rather than an appeal to authority. Furthermore, the historical accuracy of the argument can be questioned. Hitler's theistic proclaimations are well documented, and anti-religious quotes attributed to him are apparently inauthentic. He seems to have held to basic doctrines of Christianity, in spite of rather unorthodox changes, such as his belief that Jesus was an Aryan and Paul corrupted Christianity with proto-Bolshevism. Stalin was an atheist, but given that this is one of many beliefs he held, it is unclear why his actions should be attributed to his atheism. For example, though many would be surprised by this, Stalin opposed mainstream theories of evolution on the grounds that they were too capitalistic. Stalin's rejection of evolution could just as easily be named the source of his crimes as his rejection of God, and indeed his rejection of evolution arguably sheds more light on the ideological dogmatism at the heart of the Soviet regime.
If God does not exist, humans are just animals
One snappy response to this argument is "Humans are animals whether or not God exists," which has indeed been the consensus view among taxonomists since Aristotle. Though this point may seem trivial, but beneath it is the deeper point that it is hard to see how God's existing or not existing changes the status of humans. If the theist insists on claiming that human beings are worthless on their innate attributes alone, it is hard to see how God could change this situation.
Moral law requires a Lawgiver
Though we sometimes use the same words to talk about moral principles and human legislation, closer inspection calls into doubt the claim that there is a strong analogy between them. Human laws can be changed if the government wills it and follows correct procedures, but moral principles are typically thought to be unchanging. Also, it is possible to have a bad human law, but it is impossible to have a bad moral principle. In response to this second argument, it could be claimed that amoral laws are analogous to acts of a lower body that violate acts of a higher body which the lower body is responsible. This seems intuitively wrong, however: the wrong in a national law relegating part of the population to sub-human status seems very different, and more serious, than the wrong in a local law that contradicts a state
God's rewards and punishments needed to make morality in one's own self interest
Usually, this argument is not stated so baldly. A more typical statement is "we admire people who sacrifice their lives for others, but if there is no God who rewards self-sacrifice, then such people are being stupid." When the underlying assumption is stated explicitly, most people recoil. Most people do not believe that the ultimate maxim by which we should act is "look out only for your own self interest." Though such a view is technically an ethical theory (known to philosophers as ethical egoism), it is not what most people mean when they talk about morality. It seems that if ethical egoism is true, then the second premise of the moral argument is false, at least in the normally understood sense.
Absolute morality requires an absolute standard
Statements of this argument are often unclear, but it seems to rest on an equivocation of the term "absolute," in much the same way that the Lawgiver argument rests on an equivocation of the term "law." The two relevant senses here are "applicable in all cases" (a characteristic typically applied to moral principles) and "omnipotent, omniscient, etc." (a characteristic typically assigned to God). There is no reason to think that the first sense entails the second sense.
Arguments against the first premise
The Euthyphro dilemma
This is perhaps the most famous objection to the second premise. The Euthyphro dilemma is found in Plato's Euthyphro, in which Socrates asks the question, "Is the pious loved by the gods because it is pious, or is it pious because it is loved by the gods?" In layman's terms this would be, "Is that which is good commanded by God because it's good, or is it good because God commands it?" Both options are problematic for those who would claim morality is dependent upon God.
If God is free to decide what is good, and it is good by virtue of his decree, then God has no higher standard to answer to. Therefore his will may be seen as genuinely arbitrary. Although God once decreed that murder and theft are morally wrong, he might have declared the opposite just as easily, so then murder and theft would be right. This makes morality arbitrary, not what most theists mean to say in articulating the second premise of the moral argument.
If right and wrong are inherent to the action, regardless of God's decree, then God has nothing to do with the process. God doesn't set moral standards; he follows them, and is therefore irrelevant to morality (except to the extent that he can tell us things which we could not figure out for ourselves.)
An effective summary of the argument was given by Bertrand Russell:
"The point I am concerned with is that, if you are quite sure there is a difference between right and wrong, then you are then in this situation: is that difference due to God's fiat or is it not? If it is due to God's fiat, then for God himself there is no difference between right and wrong, and it is no longer a significant statement to say that God is good."
One perceived way to get out of the dilemma is to say that, although God has the freedom to command immoral acts such as rape, he would never do such a thing because it goes against his character or nature.
In response, Michael Martin has argued that this doesn't solve anything because the dilemma can be reformulated in terms of God's character: "Is God's character the way it is because it is good or is God's character good simply because it is God's character?" The structure of this modified dilemma is exactly the same as before, and it appears to be if anything harder to escape.
Moral truths as necessary truths
Richard Swinburne, a theistic philosopher, has argued that moral truths cannot depend on God because moral truths are necessary truths, existing in all possible worlds, including ones where God does not exist. This objection of Swinburne's was cited by Jeffery Jay Lowder in Lowder's debate with Phil Fernandes. Keith Yandell, another theistic philosopher, raised a similar objection in his comments on the Craig-Flew debate.
Main article: secular morality
Finally, it can in general be claimed that there is a specific, well-founded theory of morality that leaves God out of the picture. This is a complex topic and is dealt with in full by the above-linked article. One thing is worth noting here: some theists appear to think that it constitutes a valid link in the moral argument to simply demand a secular theory of morality without giving any reason to think that theistic theories are more likely to be sucessful. This is clearly fallacious, and debaters should not fall into this trap. Meta-ethics, like most areas of philosophy, has unresolved debates, but pointing to an unresolved philosophical debate is no argument for the exstience of God. To show that the moral argument is unsuccessful, one need only show that we should not accept the second premise. Full development of secular theory of morality may be helpful here, but it is not necessary.
Why Christians Cannot Account for Morality
Attribution of this article: Strong Atheism.net 
There are many reasons why Christianity is immoral, amoral, disgusting, and does not pertain to morality at all. However, I want to focus specifically on accounting for the existence of morality. Two specific points : moral autonomy and basic assumptions. p(cont).
1. As much as presuppositionalists rant against moral autonomy and freethought, we are all morally autonomous – at least, every single person able to read this blog and understand it. Ironically, we simply have no choice in this matter. Whatever standard of morality we follow, was chosen on the basis of one’s values. When the Christian asks us to surrender our moral autonomy to follow Christian doctrines, he is asking for a contradiction. To follow Christianity itself, or to continue to follow it (in the cases of childhood brainwashing) is a choice.
Christianity, then, is inscribed within a larger moral context, of which it is but one little subset. CHRISTIANITY CANNOT BE THE “FIRST CAUSE” OF MORALITY. Insofar as the Christian must, then, borrow from our secular worldviews in order to make the decision of following his religion, he is contradicting himself, and cannot account for that decision by using Christianity. Thus Christianity cannot, and necessarily so because it is a religion and not a necessity, account for its own moral validity.
The Christian decides to surrender his will on the basis of his corrupt values of submission, sacrifice, faith and opposition to the natural – just like the atheist decides to affirm his will on the basis of his own values of rationality, honesty and support for the natural. And all these values can be rationally evaluated, putting the action of “following Christianity and sacrificing some of one’s values” on an inferior moral ground to “following one’s personal values fully”.
Of course, the presuppositionalist attempts to deny moral autonomy because to do so automatically puts Christianity on such inferior moral ground. As for all collectivism, Christianity is counter to the individual, therefore sacrifice must be glorified in itself, beyond moral autonomy itself, in a higher realm of thought or ontology (the latter, in the case of God and so-called “divine morality”). p(cont).
2. Christianity cannot account for the most obvious natural-moral assumptions we hold. Assumptions such as :
•that genocide is evil.
•that we are not guilty for the crimes of our ancestors, and born evil.
•that the worth of a man is not based on his beliefs, but on his actions.
Each of these basic assumptions is contradicted by fundamental Christian beliefs. The assumption that genocide is evil, is contradicted by the Flood, the Plagues, and the Old Testament genocides are ordained by a perfectly good god. The assumption that we are not guilty of our ancestors’ crimes (and by extension, that we are separate individuals with separate actions) is contradicted by original sin. And the assumption that the worth of a man is based on his actions is contradicted by the standard of faith for salvation.
In each case, to affirm the obvious natural-moral assumption is to deny the truth of Christianity. These are all direct contradictions and there is no wiggling room for the Christian. So either the Christian rejects natural morality, which is absurd, or he keeps the contradiction, which is dishonest, or he rejects Christianity, which is the most reasonable position to take. p(cont).
3. There are many stages of morality. The first kind is order-based morality : the parents or teachers give orders, and we obey. This is a necessary stage in our development : pretty simple, so I don’t need to explain it. The second stage is natural morality.
Where does natural morality come from ? Part of it comes from biological evolution, as our attitudes and feelings are initially set because of evolutionary goals. Another part of it comes from the common observations we make as we grow up, and our process of maturation – our recognition that other minds exist, and that those minds have their own values, and later in life as a recognition of the needs of living in society together. We got them from the loving relationship we had with our parents, and later in life from the trust and love we have for other people. We get them still from our yearning for peace and plenty in ourselves, our family, our society, our world. All of these things are natural and don’t require a religion or doctrine. Teenage rebellion is also in this category, as an affirmation of one’s independence and a reaction to the previous order-based morality.
Natural morality gives us the independence, love of peace, honesty and reason we need to “graduate” to the next stage, which is rational morality. This third stage is based on the recognition that human actions have causal, measurable effects that can be used to evaluate our actions. Some of these are already present at earlier stages – for example, we learn pretty quickly that we must eat, and that not eating is bad, although we may not completely understand the whole process. But to follow rational morality is to explicitly understand the causal links between our actions and their effects, through both science and our observations of life in society.
There are four basic types of values in rational morality : physical values, spiritual (mental) values, social values and political values. There are also many virtues such as purpose, honesty, rationality, benevolence, non-coercion, Trader Principle, and so on.
Are there further stages of morality ? I think so. In my opinion, a further stage would be to be able to model social institutions and even human nature itself, with the aims of using this as a springboard for higher forms of thought. But for the moment this resides more in the realm of science-fiction.
Other formulations of the moral argument
Normativity of morality
This formulation of the moral argument relies on the assumption of normativity, that is to say, that the awareness of morality is a more or less universal experience among humans. Most people recognize that, for example, murder is wrong. From there, a theist claims that this universal awareness must come from some ultimate source, which is God.
To put it concisely:
- It appears to human beings that moral normativity exists.
- The best explanation of moral normativity is that it is grounded in God.
- Therefore God exists.
This version of the moral argument may sometimes be used by theists as red herring when responding to arguments about the moral nature of God. For instance, a person who points out the inherent cruelty of exterminating 99% of the earth's population, as in the story of Noah's ark, or takes issue with the apparent Biblical support of slavery and rape, may quickly expect to be countered with this claim:
"You recognize mass murder/slavery/rape as a bad thing, so you must have some standard to judge that against. If there was no God, then you'd have no rational reason to say that those things aren't good."
Counter-apologetic responses to normativity
- Although the awareness of SOME sort of right and wrong is apparently universal, many specific details differ across cultures and time periods. In the case of slavery, for example, the practice was once universally accepted in the southern United States, and many anti-abolitionists even quoted the Bible to justify the practice. (See the main slavery article for more details.) This indicates that morality has a strong cultural component to it, and is tied up in evolving notions of secular morality.
- Other human perceptions also have the appearance of being normative. For instance, most people agree that chocolate is "delicious," while dirt is "not delicious." By the same reasoning as the argument from normative morality, it could be said that there must be some ultimate standard for deliciousness, and that standard must be God, the ultimate tasty treat. We could use a similar argument to prove that God is the definition of the perfect homosexual lover.
- The fact that there may be an abstract standard of perfect goodness that an individual strives to achieve, does not indicate that this standard represents an existing object. For example, bowling a perfect game would yield a score of 300. However, even if no one in history had ever bowled a 300, this would still be the highest attainable score according to the rules of the game. It is quite possible to have a theoretical ideal, yet not have any concrete instance of that ideal. Therefore, we could say: "Yes, this thing that you call 'God' could be our standard for morality. However, this tells us nothing about whether or not God exists."
- Stan W. Wallace, ed. Does God Exist?: The Craig-Flew Debate. Ashgate, 2003.
- Richard Swinburne. The Existence of God. Oxford University Press, 2004.
- Richard C. Carrier. 'Hitler's Table Talk: Troubling Finds.' German Studies Review 26.3 (Oct 2003): 561-76.