Much has been made of the divide between religious ethics and secular ethics in today’s society. Debates between prominent atheists and apologetic Christians almost always mention at some point the different foundations of religious and secular ethics, and generally claim that one is better than the other. However, the distinction between the two might involve differences that you wouldn't expect.

Foundations

Many ethicists agree that the foundation of all ethics comes through evolution. Religious ethicists, on the other hand, claim that the moral rules people have come to learn through cultural evolution were predicted by God, and thus are correct at a more fundamental level than what would be natural selected for. As Descartes put it in his "Meditations," God would not intentionally mislead people, so they can trust their senses to tell them the truth. The moral sense is no different from any other senses in this fashion; if people have come to see a moral truth through evolution, then that moral truth must be true above and beyond the happenstance of that evolution.

Not all secular ethicists disagree on principle, notes the Internet Encyclopedia of Philosophy; quite a few atheistic ethicists are nevertheless moral realists, and so believe that there may in fact be a deeper truth to moral facts despite their evolutionary origins. However, the trend among professional philosophers is to accept moral antirealism, and to assert that there is no deeper meaning to moral rules than what evolution has dictated.

Personal Responsibility

Another divide between secular and religious ethics is that of personal responsibility. Secular moral realists claim that they do good for the sake of doing good, while religious moral realists tend to do good because they are instructed to be good by God, with heaven as a carrot and hell as a stick. Take, for example, the popular children’s Christmas song “Santa Claus Is Coming to Town,” which depicts Santa as actively treating people differently based on how they choose to act. Although a line in the song admonishes children to be good for goodness’ sake, the entirety of the song actively entreats children to do good “or else.” While doing good to do good is common among secularists, it appears that many religious adherents are instead focusing on the “or else” portion, notes the Reasonable Faith website.

Authoritative Texts

For most Christians, the Bible is supposed to be the source of what is good, while for secularists, authoritative works change over time as society adapts its views on cultural norms. This difference is highlighted in the book, "The Science of Good and Evil: Why We Are Moral" by Michael Shermer: a group of religious children is presented with the Biblical story of Joshua at Jericho. When told that Joshua’s army had killed all the women, men, livestock and even pets in the city, 66 percent of the children approved of the action merely because it was sanctioned by God. A control group was presented with the identical story, except Joshua’s name was replaced with that of a fictional “General Lin,” and the story was set not in Jericho but a “Chinese kingdom 3,000 years ago.” Only 7 percent of the control group approved of this latter story. The author concludes that religion serves as a sort of off switch for morality in the human brain.

Arbitrariness

Plato argued that if you follow an ethical rule because God indicates it is good to do so, then you are not truly being ethical. The moral person would instead do that thing because it is good, and not because God informed her that it is good. Hence, if ethics is based in religion, then ethics is ultimately meaningless.

Plato’s argument starts by pointing out that the gods say that some things are good. Either the gods deem them good because they have better knowledge of what is good and are therefore relating that knowledge to people, or the gods deem them good because they are the source of the goodness. If the former, then ethics does not arise from divine influences; moral truths are true regardless of what God happens to relate to people. Yet if the latter is true, then whatever the gods happen to choose as good is entirely arbitrary. According to Plato, this means that either ethics is ultimately secular and meaningful, or it is religious and ultimately arbitrary.