Troy Jollimore in Aeon:
[W]hy do so many people believe that morality needs to be grounded in religion, when the arguments in favour of that view are so unconvincing? I suspect that something else is going on, and that in most cases these arguments are just rationalisations for the belief that morality depends on faith in God. The actual explanation, I believe, is something else.
The reality is, no system of secular ethics has managed to displace religious approaches to ethics in the contemporary popular imagination. It is worth asking why. We can start with the fact that the secular approaches that have dominated Western thought since the Enlightenment tend to share certain features. The two most significant post-Enlightenment secular theories are those derived from the work of the Prussian philosopher Immanuel Kant, and utilitarianism, which originates in the work of the British philosophers Jeremy Bentham, James Mill, and John Stuart Mill.
Utilitarian ethics claims that the right thing to do is always the one that will maximise happiness or well-being among the general population. The answers to our moral questions are, thus, to be determined by empirical research — what will make people happiest or best-off, on the whole? Kantian ethics — to put a highly complex theory into a very small nutshell — says that reason commands us to behave morally. Moral truths are, in essence, logical truths, so that the content of morality can and ought to be determined from the philosopher’s armchair.