Contrary to all those who consider Aquinas to be, if not the father, at least the instigator of Cartesian rationalism Fergus Kerr argues that St. Thomas did not propose rational proofs in order to prove the existence of God through unaided human reason:
That God’s existence is something that does need to be argued for, Thomas holds, is based on the fact that we do not know what God’s nature is – as the doctrine of the divine simplicity will say. In other words, the God who is present in the world as source and goal of all things is so much more mysterious than those who see signs of divinity everywhere believe that argument is required. The point of insisting that argument for God’s existence is required is, then, not to convince hypothetical open-minded atheists, or even to persuade ‘fools’, so much as to deepen and enhance the mystery of the hidden God. From the start, the ‘theistic proofs’ are the first lesson in Thomas’s negative theology. Far from being an exercise in rationalistic apologetics, the purpose of arguing for God’s existence is to protect God’s transcendence (After Aquinas: Versions of Thomism, p. 58 )
So, the “Five Ways” are not proofs for Christians to debate with their atheist buddies. They are not meant to infallibly prove God’s existence. Quite the opposite; they are meant to demonstrate God’s incomprehensibility. Furthermore, Aquinas’s negative theology can be seen in the manner in which he describes God, as Ipsum Esse Subsistens (Being in and of itself). The Christian God cannot be known univocally and cannot be demonstrated apart from the authority of Sacred Scripture because he transcends all analogies. Brian Davies notes, concerning the analogy of being:
… it makes sense to deny that God, like a creature, has being. Rather, so he [Thomas] suggests, we might speak of God as Being Itself. His meaning is not that God is an is-ing kind of thing. His point is essentially a negative one. Since the claim that God is Ipsum Esse Subsistens seems to be telling us what God is, one might expect Aquinas to defend it in an account of God’s properties or attributes. But that is not what he does. We cannot, he argues, know what God is. We must content ourselves with considering “the ways in which God does not exist, rather than the ways in which he does.” And it is here that his talk of God as Ipsum Esse Subsistens comes in. It is part of an account of ways in which God does not exist. Its chief purpose is to deny that God is a creature. As some authors would say, it is an exercise in negative theology. (Thomas Aquinas: Contemporary Philosophical Perspectives, p. 11)
For Thomas to hold that God is Being is for him to hold that God is not being. The “Five Ways” seen in this light are just as much the “Five Ways” in which God does not exist as they are the “Five Ways” in which he does by necessity.