Let's talk about Ethics now. Most of us have, whether it comes from Instinct, Reason, or Culture, a strong belief that certain acts and behaviors are morally right and therefore obligatory or commendable, while others are morally wrong and therefore forbidden or reprehensible. Until we are exposed to certain philosophical questions, we tend to assume that this is just part of how the world is: that there is right and wrong and that one can persuade other people of it. “It's not fair!” say small children to their parents. There is, admittedly, some disagreement about what morality says (just as there is disagreement about everything else), but there is also a fair amount of common ground.
Once people get exposed to Philosophy (even if only in the form of a shallow cultural relativism common among college Freshmen), it is natural to question whether this sense of ethics is grounded in the actual objective nature of reality. Or is it is purely subjective? In the latter case, one should probably bite the bullet and say that, other than as descriptions of our own psychology, moral facts don't really exist at all (Ethical Nihilism), and it is a mistake to think that there is somehow a fact of the matter about e.g. whether murder or adultery are really wrong.
It seems to me that the belief that e.g. “murder isn't really wrong” is morally abhorrent, and that anyone who really disbelieved in the truth of ethics (though perhaps it is not fully possible) would be leaving behind an important part of their human heritage. Just as a person whose left and right brain hemispheres have been severed is a defective or damaged human specimen, so the person whose heart and mind have been severed by moral relativism fails to be fully humane. At the very least, Ethical Nihilism hardly seems likely to inspire moral excellence. Even the moral duty to believe what is true would in principle be undermined by it. But this, however important it may be practically, is a moral argument in favor of morality, and those who do not accept this vision of humanity may accuse me of arguing in a circle. Instead, let's ask what could ground ethical truths.
There are many views which have been held about Meta-Ethical theory. For simplicity let's consider four main ones: which we might call, with some degree of over-simplification, the Protagorean view, the Kantian view, the Aristotelian view, and the Platonic view. All but the first of these views attempt to ground morality in some sort of objective reality, but in different ways.
The Protagorean view is that Ethics is grounded in nothing more than one's own personal subjective opinion. That opinion may be partly determined by cultural or biological factors, but there is nothing inherently good or bad in accepting or defying one's heritage: whatever you want to do is best. If somebody sincerely believes a different sort of ethical system which permits say revenge and genocide, they aren't really any better or worse than anyone else, just different. We judge them to be bad, but then again they judge us to be bad. Once you decide to pursue a particular goal, you can ask whether your means are well-chosen to suit your ends, but your ends are really up to you. Thus, the wise man who wishes to live in a peaceful city might perpetuate myths which help other people to be virtuous, but he won't believe any of them himself.
The Kantian view is that Reason tells us that certain things are right and wrong; that the reasoning mind can know the truths of Ethics much as we know the truths of Mathematics, by deducing them from self-evident first principles. Kant himself had an argument for something called the Categorical Imperative which was roughly like the Golden Rule, and he claimed that this was equivalent to treating other people as ends rather than means. Nobody much accepts his specific arguments anymore, but newspaper ethics columnists still have a broadly Kantian mindset, perhaps because it takes them more or less where they want to go, given the current tendency among democracies to reformulate all ethical questions in terms of “Human Rights” vested in autonomous (self-governing) individuals.
The Aristotelian view is that Ethics is grounded in human nature. That is, all living creatures have some sort of intrinsic goal, purpose or end (τελος) which is what it means for that plant or animal to flourish, fully developing its nature in the way that is good for it. Evil would be a perversion or corruption of a thing's nature, not something which has an independent existence apart from the telos of a thing. For us as humans, Ethics consists of identifying the requirements of human nature and cultivating habits which help to promote that flourishing. Since we are sexual beings, part of our good is directed towards reproducing ourselves, and since we are “political animals”, another part of our nature consists in promoting benevolence towards others, but the highest and noblest aim (according to Aristotle) is to develop our rational nature, which flourishes when we pursue philosophy. Each person has their own individual telos (what's good for you is not necessarily good for me)—which is however objective, since it is grounded in a more-or-less universal human nature.
(Some people might think that this Aristotelian view that biological organisms have purposes, i.e “final causes” is undermined by Darwinian evolution, but this is contestable. Did Darwin eliminate purposes from the biological world, or did he explain their existence? In any case, we are not here dealing with the Design question of how organisms like us came to exist, but with the quite distinct question of what our ethical significance is, now that we do exist.)
The Platonic view is that there is a transcendental principle called “the Good” or “goodness itself”, which acts as the standard or judge for all other things. Thus, as in the Aristotelian view, goodness is based on the nature of things, but now it is a property of the fundamental nature of existence. All things, to the extent that they exist, participate to a greater or lesser extent in goodness. They are thus good only in a derivative sense, by participating in goodness. The philosopher begins by appreciating the beauty or virtue in visible realities, but ascends from there to appreciating the primary goodness in that which is Beauty or Virtue itself. Thus, in this view, the fundamental nature of reality requires us to be benevolent to others; although the precise set of actions to be performed are doubtless (as in the Aristotelian view) dependent on the precise details of human nature (it is kind to give a beggar bread rather than cyanide because of the nature of human biology, but what kindness is does not depend on that).
Of these four views, Platonism is particularly conducive to arguing for Ethical Monotheism, due to its ascribing all goodness to the fundamental nature of things. On the Aristotelian view, one can still attempt to trace the teloi back to their ultimate goal, much as the Cosmological Argument traces causes back to their ultimate cause. (By rights this ought to be called the Teleological Argument, but unfortunately that term is usually taken to be synonymous with the Argument from Design.) If Ethics can be deduced rationally as in the Kantian system, then one can at least deduce that if the Universe originates from something like a mind, that mind should also be able to appreciate ethical truths. But on the Protagorean view, the Argument from Ethics is dead in the water and can go nowhere.