Often I reference Natural Theology on my forum/blog, but I do not often give an explanation for what it is in a basic sense. So with this post I hope to quickly remedy that by providing a basic definition of what natural theology entails, and who was one of its most famous and early proponents. Millard Erickson in his systematic theology Introducing Christian Doctrine has written this in description and definition of what natural theology is at a basic level:
The core of natural theology is the idea that it is possible, without prior commitment of faith to the beliefs of Christianity, and without relying upon any special authority, such as an institution (the church) or a document (the Bible), to come to a genuine knowledge of God on the basis of reason alone. Reason here refers to the human capacity to discover, understand, interpret, and evaluate the truth.
Perhaps the outstanding example of natural theology in the history of the church is the massive effort of Thomas Aquinas. According to Thomas, all truth belongs to one of two realms. The lower realm is the realm of nature, the higher the realm of grace. While the claims pertaining to the upper realm must be accepted on authority, those pertaining to the lower realm may be known by reason.
This seems probably pretty vanilla for most evangelical Christians to digest, and something most are pretty familiar with; in fact it seems intuitive, does it not? But I reject this; I reject the idea, along with Karl Barth and against Thomas Aquinas that people in general can come to a genuine knowledge of God apart from God’s particular Self-revelation in Jesus Christ. Nevertheless, most of the Western tradition of Christianity (of which almost all of my readers are participants of, i.e. the Western trad) affirms natural theology. It affirms conceiving of God, categorically by way of employing philosophical reflection as determinative for how we supply ourselves with a grammar that articulates God. I reject this though. So does Karl Barth, and here is why (and this is a full length quote, so hang on),
“The content of revelation is wholly God.” The point here is simply that God is not just half revealed, so that another part of his being or attributes or acts will have to remain hidden or will have to be imparted in some other way than by revelation. As regards the second possibility, we have to think especially of the increasing role played in Protestant theology from the end of the 16th century by what is variously called natural theology or revelation or religion (as distinct from the supernatural or Christian revelation). Natural revelation includes not only the voice of God in nature, as the name indicates, but also such things as conscience, the moral light of nature, religious feelings or dispositions or tendencies in us, mathematical and philosophical axioms, what better pagans know about the existence and unity of God, and the creation and overruling of the world by him, and non-Christian analogies even to such central Christian mysteries as the Trinity and the incarnation. Theologians usually regarded and employed this natural revelation as a good and useful narthex or first stage on the way to the true Christian revelation. The older Reformed theology in particular attached high importance to this preliminary structure. According to A. Schweizer one might even see in it one of the most valuable features of Reformed theology. It was given a place of honor in the 19th century both in the first part of Schleiermacher’s Christian Faith and in Schweizer’s own Glaubenslehre. Vestigia terrent! For my part, although I am Reformed, I want no part of it. You will not be surprised at this in view of what I have said earlier. Either God speaks, or he does not. But he does not speak more or less, or partially, or in pieces, here a bit and there a bit. This is a contradiction in terms, an anthropomorphism, a basic naturalizing of revelation which fits Schleiermacher very well, but which ought not to have found any place among the older Reformed. Calvin at the end of the discussion in the first chapters of the Institutes was perspicacious enough to raise the whole question again, to oppose the Christian knowledge of God dialectically to natural knowledge, and to proceed as though there were only the former. And even in Thomas Aquinas the insights one can gain into God’s nature apart from revelation have the significance only of a possible and necessary ancillary construction that pays secondary honor to the truth of revelation. If God speaks, then God speaks, and we have to do with the one Logos that the prophets and apostles received, the one revelation in the incarnation which the people of the Bible know and attest as either promised or manifested. Nothing prevents us, and much urgently inclines us to suppose that others, too, might have had a share, and might still have a share, in the same divine answer. We do well at this point to confess the free and broad outlook of Aquinas when he said that all truth, no matter who speaks it, is of the Holy Spirit, or of Zwingli when he said that whoever speaks truth speaks of God. But the truth must then be understood as the one totality of truth, and the words “Holy Spirit” and “God” must be taken in a pregnant sense. Truth that really goes back to God cannot be a particle of truth. It is either the whole truth or it does not go back to God and is not revelation at all.
Interesting how Barth constructively engages with Aquinas, while at the same time ardently rejecting what Thomas became largely known for: natural theology.
What is the practical implication if we follow natural theology? Does it affect the way we think of God if we seek God and his ways, his perfections from nature before we encounter and meet him in Jesus Christ? What of the Old Testament, someone might ask, don’t we see older people of God engaging in natural theology? Isn’t God ‘progressively’ unfolded for us in the Old Testament before we ever meet him in Christ, and don’t we infer things about God in the Old Testament that are a result of reflecting upon him in his activity in nature (like creation, etc.)?
The above might represent challenges to Barth’s position against natural theology, and in fact, there are even more nuanced Greek ones that appeal to the Logos and His place in the taxis or order of nature itself (the Eastern Orthodox are fond of going this way).
I think you “non-specialist” Christian out there, you would be very surprised if you dug deeper how entrenched your understanding of God is by natural theology. I think natural theology ends up in idolatry, and it elevates humanity’s capacity to know God that in the end supplants a need for God by a continual need for us and our intellects in order to create space and grammar for God without allowing God’s own life to determine the shape of the grammar he would have us use in order to understand him under the force and compelling reality of his own life.
 Millard J. Erickson, Introducing Christian Doctrine (Grand Rapids, Michigan: Baker Book House, 1996), 35.
 I have used the picture of Adolf Hitler in this post because it was the context within which Barth wrote and wrote against. Hitler and the Reich would have represented a socio-cultural expression and logical/theological conclusion to where natural theology when consistently held to could potentially lead. In other words, natural theology is an ‘under the sun’ theology such that what ‘is’ (for the natural theologian) is what ‘ought’ to be. If we start any other place other than God in Jesus Christ as the ‘is’ and thus the ‘ought’, if we start from below, like Hitler did, if we start from our own thinking and reflection upon nature, consistent with this, in an extreme but logical form, might result in something like the Holocaust, or even the Apartheid of South Africa. So I appeal to Hitler not to shut any further arguments down, but to illustrate Barth’s context, and why he with such fervor abhorred natural theology as the following quote from him will demonstrate.
 Karl Barth, The Gottingen Dogmatics: Instruction in the Christian Religion (Grand Rapids, Michigan: Eerdmans Publishing Company, 1991), 91-2.
 Also see my chapter, “Analogia Fidei or Analogia Entis: Either Through Christ or Through Nature,” in Evangelical Calvinism: Essays Resourcing the Continuing Reformation of the Church, eds. Myk Habets and Bobby Grow (Eugene, OR: Pickwick Publications, 2012), 94-113. Also I should note, that I am not suggesting that we are not part of the equation, and that we do not or should not engage with every ounce of capacity the Lord has given us; instead with my point here, I am suggesting that we need to, by way of order, allow God’s crystalline voice spoken most evidently and clearly in his dearly beloved Son, Jesus Christ by the breath of the Holy Spirit, to be the voice we seek first. There is no voice in nature, apart from the One who first gave nature by fiat in and for Jesus Christ (Col. 1.15ff; Rev. 21–22; etc.). He alone exegetes, explains the Father, the God-head for us. If domain of the Word, of Jesus Christ, is inclusive of nature, then it behooves us to start with her King, and think from there. This whole post, this whole consideration comes back to a matter of theological methodology–and yet I cannot stress how important this at a most basic level, that is, knowing God.