In The Grammar of Assent, Cardinal Newman refers to the pre-Christian religions and their awareness of the supernatural. We know that “natural revelation” permitted the ancients to recognize in the world around them the necessity of a creator, the spirituality of the soul and the existence of moral obligations. What is perhaps less obvious, Newman explains, is that concepts of guilt, sin, punishment, expiation, and salvation are not recent inventions but ineradicable features of the human mind which manifested themselves throughout pre-Christian cultures. At the same time, paganism succumbed to fatalism and was far more pessimistic than anything Christianity had to offer. More importantly, it was plagued by contradictions and a lack of theological closure. Says Newman
Revelation begins where Natural Religion fails. The Religion of Nature is a mere inchoation, and needs a complement, — it can have but one complement, and that very complement is Christianity. Natural religion is based upon the sense of sin; it recognizes the disease, but it cannot find, it does but look out for the remedy. That remedy, both for guilt and for moral impotence, is found in the central doctrine of Revelation, the Mediation of Christ.
Hence it would be wrong to think of supernatural revelation as a mere supplement or pious afterthought. Speaking from my own experience, even the best Greek and Roman philosophers proposed ideals which were impossible to achieve according to their own systems. Someone like Aristotle writes with an awareness of our flawed nature and of the necessity for virtuous living, but what he cannot supply is the explanation for our moral frailty or the metaphysical necessity for our moral virtue.