Aquinas’ “moral argument” might not be what you expect to find.
Thomas Aquinas is famous for – among other things – his five arguments for Christian theism, arguments called “the five ways” (quinque viae). The first way is the argument from the unmoved mover. The second way is the argument from the first cause (commonly called the cosmological argument). The third way is the argument from contingency and the fifth way is the argument from purpose (not from design, contra Richard Dawkins), namely the teleological argument.
What is the fourth way? In popular apologetics (and anti-apologetics) it is often referred to as the “moral argument” for God’s existence. However, if it is a moral argument, it is unlike any other moral argument you’re likely to hear. In Aquinas’ words:
The fourth way is taken from the gradation to be found in things. Among beings there are some more and some less good, true, noble and the like. But “more” and “less” are predicated of different things, according as they resemble in their different ways something which is the maximum, as a thing is said to be hotter according as it more nearly resembles that which is hottest; so that there is something which is truest, something best, something noblest and, consequently, something which is uttermost being; for those things that are greatest in truth are greatest in being, as it is written in Metaphysics. ii. Now the maximum in any genus is the cause of all in that genus; as fire, which is the maximum heat, is the cause of all hot things. Therefore there must also be something which is to all beings the cause of their being, goodness, and every other perfection; and this we call God.
If after reading this you’re wondering “where’s the moral argument in that?” you’re not alone. I’m not faulting the argument itself in saying that. It’s an argument in its own right, it’s just not a moral argument.
The moral argument as we know it was overtly introduced into philosophy of religion by Immanuel Kant. His was an argument that sought to explain the existence of moral duty, which is what we’ve come to expect of moral arguments. Moral duties are lacking in Aquinas’ fourth way. Aquinas’ argument is a platonic argument (an argument influenced by Plato). In Plato’s thought, things that exist in the world are shadows of ideal things that exist in the world of forms or ideas. For example, a thing is a good train to the extent that it resembles the train, a blueprint of the perfect train, a train existing in the world of forms. This is the form of a train. Likewise, things that have goodness in this world, the “world of sight” as Plato put it, are good to the extent that they resemble the good, the perfect form or idea of goodness in concrete form (concrete in the sense that Plato spoke of “the good” rather than the more abstract “goodness”).
What Aquinas is really arguing here is that something perfect exists. After all, he reasons, degrees of perfection exist in things around us, but if degrees of perfection exist, then something maximally perfect exists, something that all other things resemble to the extent that they exhibit perfection.
Maybe you could re-tool the argument into a moral argument along the following lines: The best people in the world are righteous, and the worst are wicked, and this is a matter of fact. But if we measure things as righteous or wicked then there must be some standard according to which we measure them, something perfectly righteous that people can either resemble or fail to resemble. And this perfectly righteous thing is God. Make no mistake, this was not Aquinas’ argument. It also would not be the best version of the moral argument, even if Aquinas had argued this way. I’m not even sure that it’s sound, but if you want a moral argument closely modelled on Aquinas’ fourth way, that’s about as good as it gets.
NB: My opinion is subject to change without notice.