The analogia entis, or ‘analogy of being,’ has been a topic of interest for me almost from the moment I heard the term; linguistically the language itself sounds cool, but that’s where the coolness level leaves 
off for me. As a general introduction to Thomas Aquinas’ thinking on analogy of being the Stanford Encyclopedia of Philosophy will suffice:
Despite the vast modern literature devoted to Aquinas’s theory of analogy, he has very little to say about analogy as such. He uses a general division into equivocal, univocal, and analogical uses of terms, and he presents both of the threefold divisions of analogy mentioned in the previous section, but he offers no prolonged discussion, and he writes as if he is simply using the divisions, definitions, and examples with which everyone is familiar. His importance lies in the way he used this standard material to present an account of the divine names, or how it is we can meaningfully use such words as ‘good’ and ‘wise’ of God.
The background to this account has to be understood in terms of Aquinas’s theology and metaphysics. Three doctrines are particularly important. First, there is the distinction between being existent, good, wise, and so on, essentially, and being existent, good, wise, and so on, by participation. God is whatever he is essentially, and as a result he is existence itself, goodness itself, wisdom itself. Creatures are existent, good, wise, only by sharing in God’s existence, goodness, and wisdom, and this sharing has three features. It involves a separation between the creature and what the creature has; it involves a deficient similarity to God; and it is based on a causal relation. What is essentially existent or good is the cause of what has existence or goodness by participation. Second, there is the general doctrine of causality according to which every agent produces something like itself. Agent causality and similarity cannot be separated. Third, there is Aquinas’s belief that we are indeed entitled to claim that God is existent, good, wise, and so on, even though we cannot know his essence.[1]
It is true that Barth’s nearest referent when referring to the analogy of being in particular, was 20th century Catholic theologian Erich Przywara; but when thinking these things in general, as he is doing in our consideration of his thought, Thomas Aquinas’ version of the analogy would be in the cross-hairs just as much as Przywara or anyone else.
The Stanford introduction to Aquinas’ thinking does not mention the language of analogia entis, but its description of analogy in Aquinas sufficiently sketches the primary entailments of the entis in the Dumb Ox’s thinking. Of note, for our purposes, is the conception of God that is presupposed in Aquinas’ understanding. For Aquinas, along with Aristotle, God is ‘first cause,’ the ‘unmoved mover,’ the ‘actual infinite.’ We see this being alluded to in Stanford’s when it refers to the metaphysical aspects of God’s being, as if these simply are the standards of who God is. But how are these standards arrived at; are they based on revelation or philosophical reflection? The answer: philosophical reflection. Further, the Stanford description also alludes to a theory of causation inherent to this unmoved mover’s capacity to relate to things (like the world) that are not inherent to its inner being. As the first causer of all that is, in the Aristotelian/Aquinas schema, roughly stated: all of the subsequent causes, vis-à-vis God, can be traced back to the first cause by reference to the hierarchy of being built into the strata of all created reality. This is an important piece of the Thomist analogia being: if all reality is interconnected within a chain-of-being, it is logical to deduce that created being, at some [analogous] level can infer its primary cause from within its own caused-self.
In brief: the analogy of being refers us to the way that some philosophers and theologians have attempted to construct a theory of a knowledge of God that honors the independence of creaturely nature, as that is attenuated by God’s grace in His Self-revelation. In other words: in this frame, knowledge of God can be arrived at by simply reflecting upon the nature of things in the created order; primary of which is reflection upon human nature as the imago Dei, and how its existence finds its being elsewhere in the ‘pure being’ of God. But it is because of these elements, these prior commitments and realities, that Aquinas et al. believed that human beings could reason themselves to some general level knowledges of the Creator’s pure being, as it were.
Along with Karl Barth, I consider this framework for knowledge of God: anti-Christ. Why? Because it presumes to speak of God before God has spoken for Himself to us in Jesus Christ. This is not to say that a participatory knowledge of God is out of bounds (which the analogy of being believes is appropriate as well), but that the way we conceive of God in the first place, at a first order level, must be dictated strictly by God’s Logos for us (cf. Jn 1.1; 18 etc.). The Christian God is not a product of a general human conception of godness; the Christian God comes to us within the scandal of particularity / within the scandal of the cross. Barth points it this way (at length):
We must not overlook the fact that the moderate doctrine of analogy in natural theology, as it has been and is represented in particular in the Catholic Church, stands in the closest material and historical connexion with the Liberalism which, under appeal to God’s omnipotence, affirms all analogies. Even if it is sanctified by the teaching office of the Church, it is still an arbitrariness, grounded only in philosophy, that Catholicism will not allow Christian thinking and Christian language to draw from the analogia entis affirmed by it the consequence of a general analogy of the world to God. On the other hand, Liberalism shows a basic readiness in almost every connexion to discover new analogies in the world, and, whether it knows it or not, it stands in only too great need of the corrective of a philosophical arbitrariness (and a teaching office to sanctify it?), and necessarily evokes at least a desire for it. Genuine proclamation is not possible on the basis of the opinion that we have to reckon with an analogy of human views, concepts and words, which may be established apart from God’s revelation, and therefore on the basis of the doctrine of analogy in natural theology. It is only possible where the analogy is understood as the work and proposition of revelation itself. Genuine proclamation must speak particularly and therefore restrictedly. It must be aware why it says this and does not say that; why it says this in one way and that in another. But is particularity must not be abandoned to an arbitrary philosophy, to the chances and changes of philosophies, and finally to the dictates of a teaching office. If it is going to be proclamation of God, it must rest on the choice made by God Himself. For this very reason it must always be bound to God’s revelation, and must always be the exposition of the revelation of God. Even indirectly it must not become the self-exposition of man, or the exposition of the revelation of God under the presupposition and according to the measure of a preceding self-exposition of man. It is exposition of the revelation of God when it keeps to the human words which are placed at our disposal as we are confronted by God’s revelation, and which are therefore designed as serviceable for this employment; when it follows the freedom in which God bestows His grace upon man generally and therefore upon his human views, concepts and words. It will then have something definite to say, and that with a good conscience, with the promise of relevance, i.e., of standing in a real relationship to the reality proclaimed by it, and with the justified claim and well-grounded prospect of obtaining a hearing.[2]
Folks involved in the theology of retrieval, currently, will immediately balk at this; but ironically, they do so from the very premise under critique: i.e. natural theology. In other words, we notice how Barth takes aim at the ad hoc ‘teaching office’ of the Catholic Church; as he does, more generally, he posits that all generalizations of God in the world at large have some sort of delimiting ‘teaching office’ that sanctions this or that particular notion of ‘otherness’ as an organizing or regulative principle per its broader knowledge-system (for naturalists it would be the ‘teaching office’ of the scientific guild). The point Barth is (rightly!) militating against is that for the Christian there can be no philosophically derived notion of God that serves as God’s posterior in regard to our knowledge of Him.
Much more must and ought to be said, but, alas, this is a blog post! Barth’s battle, and those foolish enough (like me) to take it on as their own, is mostly a loosing battle among the conservative evangelically Reformed communities I inhabit. What this indicates is that such communities are slavishly bound to a natural system of theological discourse that ultimately is tied into the Church’s determination about God rather than being open to God’s disruptive voice and grace confronting them anew and afresh in Jesus Christ. This is a tragedy I hope to teach my children, and all those who have eyes to see and ears to hear, to avoid with a hastened repentance. Soli Deo Gloria
[1] Stanford Encyclopedia of Philosophy.
[2] Karl Barth, Church Dogmatics II/1 §27 The Doctrine of God: Study Edition (London: T&T Clark, 2009), 229-30.