Ed Feser concludes his book Aquinas with the following comment about the relation between the Angelic Doctor and modernity:
In both its metaphysical and theological commitments, Aquinas’s system of ethics is, like the rest of his philosophy, obviously radically at odds with the assumptions typically made by contemporary moral philosophers. But the main difference may lie in something other than a disagreement over this or that particular ontological thesis or argument for God’s existence, in basic ethos rather than intellectual orientation. The spirit of modern moral philosophy is perhaps summed up best in Kant’s famous characterization of human beings as “ends in themselves” and “self-legislators.” This sort of talk would sound blasphemous and even mad to Aquinas, for whom God alone, as the “first cause and last end of all things,” could possibly be said to be the source of moral law and an end in himself. (ST I-II.62.1, as translated by Pegis in Basic Writings of Saint Thomas Aquinas). For Aquinas, we are not here for ourselves, but for the glory of God, and precisely because this is the end set for us by nature, it is in him alone that we can find our true happiness. And it must be emphasized that, as with the other themes we’ve explored in this book, he takes this conclusion to be a matter, not of faith, but of reason itself.
Therein lies the sting of Aquinas’ challenge to modernity.