What is “natural theology,” and why should we reject it? At a very fundamental level natural theology holds that it is possible for humans to ‘know’ God at some level by reasoning from God’s works (effects) in creation back to him (the Creator) through a chain of inter-locked being (albeit an asymmetrical relationship from finite to infinite as ultimate). Thomas Aquinas is one of the best known advocates of natural theology and for a subset of that known as the analogia entis (‘analogy of being’ which I just described). Here is an example of Thomas’ thinking when he applies it to a discussion he is starting in his Summa Theologica on Divine Simplicity:
When we know that something exists, it still remains to inquire into the manner of existence, in order to know what it is. But we cannot inquire into the manner in which God exists. We can inquire only into the manner in which he does not exist, since we cannot know of God what he is, but only what he is not. We must therefore consider how God does not exist, how we know him, and how we name him. The manner in which God does not exist can be shown by excluding what is incompatible with God, such composition, movement, and the like….
In medieval theology the method Aquinas is using is known as the via negativa (‘the negative way’ which is a process of thought, in this case about God, via negation. So, for example: we are finite, therefore God is infinite; we are creative, therefore God is Creator, etc.). It presumes upon the Aristotelian idea that God is the Unmoved Mover, and that everything else subsequently (in creation) that moves has its movement given to it by God in this kind of unbroken chain of what Thomas F. Torrance calls logico-causal necessitarianism. If this is so, according to the thought of Thomas (Aquinas), it follows logically then that we can reason about God from our own being and circumstance in relation to his as the Ultimate pure being; and that this capacity remains latent and stable within creation/nature itself (apart from Grace).
Karl Barth demurs stringently from this proposal. He calls this kind of thinking, in the strongest of ways, heretical and even goes so far as calling it anti-Christ, and a primary reason for not being part of the Roman Catholic Church; note:
I regard the analogia entis as an invention of the antichrist, and I consider that because of it one cannot become a Roman Catholic. In saying this, I wish also to say that any other grounds which one may think to have for not becoming a Roman Catholic are, in my opinion, inadequate and are not to be taken seriously.
His reasoning for such a strong stance is this [according to his Dutch commentator, G.C. Berkouwer] (and this is what I want to emphasize in this post):
By means of a criticism of natural theology in which the principle of the analogia entis dominates, Barth thought to be able to point out clearly the error which threatens here. The issue that he posits here is the fundamental question about the receptiveness or non-receptiveness of man to God’s revelation. According to natural theology, man has within himself, specifically in his reason, the possibility of understanding God’s revelation, namely the revelation of God in created reality. Because there is an analogy between God’s being and man’s being, man can by way of conclusion come to a true knowledge of God. Even though this natural knowledge of reason is not adequate, it is, for all that, true knowledge. On basis of an objective state of affairs, man can, quite apart from grace, achieve knowledge of God.
Against this conception Barth protests. He posits with the greatest emphasis that we can achieve knowledge of God only through “a reaching out, which has taken place and takes place from the side of God.” According to Barth, there is no possibility of the knowability of God or of the knowledge of God apart from this “reaching out.” There is no analogy on basis of which, beginning with ourselves, we can come to a knowledge of God’s being.
Not only do we not know God as Reconciler, as Lord and Savior, but we do not know Him either as Creator. There is no “pre-understanding” (on the basis of an analogy of being) in which room is later given to God’s grace. In no single respect can we speak of “a prior knowledge about creation which we have of ourselves.” It is only God’s grace and mercy that make it possible for us to know God also as Creator….
The basic point of concern is, as Barth conceives of it, that given our situation in a Fallen world where we aren’t even able to seek after God (Rom. 3), we, for all intents and purposes live in a closed system; i.e. we cannot get out of ourselves by ourselves. And so if this is the case, we will never be able to adequately or accurately conceive of a God this way other than conceiving of one who is categorically a projection of ourselves (so the ‘god of the philosophers’).
The practical import of this seems pretty clear. If we are going to genuinely have a real knowledge of God we cannot do anything but rely on God’s Self-revelation alone; not as an aide to what we’ve already conceived of God prior to his revelation, not as a completion of it (so Thomas’ axiom of ‘grace perfecting nature’), but as a disruptive reality of grace that so breaks in upon us that it reorientates us, reconciles, and in the process provides true revelation of God Immanuel for us in Christ.
 Thomas Aquinas, Summa Theologica, I, Q.3, Art.1
 Karl Barth, KD I/1, p. vii in G.C. Berkouwer, The Triumph of Grace in the Theology of Karl Barth (Grand Rapids, Michigan: W.M.B. Eerdmans Publishing Company, 1956), 180.
 G.C. Berkouwer, The Triumph of Grace in the Theology of Karl Barth (Grand Rapids, Michigan: W.M.B. Eerdmans Publishing Company, 1956), 183-84.