One of Barth’s primary objections to natural theology — i.e. proposed knowledge of God derived through reason or the created order rather than direct revelation — is that such a thought process does not lead us to the knowledge of God revealed in Christ. Natural theology is seemingly more a matter of metaphysics than of theology.
For example, the cosmological argument from contingency may give us a picture of God as the necessary being upon which all contingencies (e.g. the universe) rely for their existence. However, such a derived definition of God as a necessary being does not lead us to the God revealed through Christ and witnessed to by scripture: namely, that God is a Redeemer, Reconciler, Liberator, Savior, Forgiver, and incarnate in Jesus. If God’s identity is truly and necessarily that of a Redeemer, etc., and the argument from contingency fails to demonstrate or mention such an identity, then the argument from contingency does not lead us to knowledge of God.
The other threat Barth was weary of was the temptation to redefine the revealed God as identical to a god developed by a metaphysical schema. An illustration he mentions in the Church Dogmatics is Aristotle. Popular in church history (particularly in the medieval era and still appealed to today) was to use Aristotelian philosophies of being and perfection to understand the nature of God. Aristotle (roughly speaking) conceived of perfect being as immutable, unchanging, atemporal, static, and wholly unrelated to the world. Such a concept of the nature of God, for Barth, was unsatisfactory. If God has the nature of the revelation in Christ and witnessed to by scripture — such that he is a Redeemer, etc. and one who actively reveals himself to us — then such an Aristotelian conception of God is not the God of Christ. It may, in fact, be an idol.
Thus, for Barth, knowledge of God must primarily come through a revelation from God himself via an act of grace.
These objections are powerful and should be thoroughly understood by any Christian contemplating the role of metaphysics and natural theology to Christian theology.
However, I think natural theology may still be conducted even while accepting and integrating Barth’s objections.
First, one of the primary purposes of natural theology is to demonstrate that religious belief is rational and reasonable. A system of thought permeated and polluted with logical contradictions could end up being little more than nonsense. A theology riddled with logical contradictions and fallacies is a theology doomed to failure. Contrarily, many individuals may be more open to accepting Christianity if they see that it’s reasonable to do so.
Additionally, natural theology in certain situations can provide a ‘neutral ground’ whereby a Christian can interact with a non-Christian who may have little-to-no previous acquaintance with religious belief or Christian language.
Natural theology can also be a useful tool to provide analogies that can dissuade certain harmful or improper conceptions of God. For example, it is true that the cosmological argument from contingency does not give us the God revealed in Christ. However, it does direct us to an understanding of God as the necessary, creative ground of all being — a God who is ‘totally other’ from all created (contingent) entities. (The notion of God as totally other was a favored notion of Barth.) Such an insight into the nature of God, although only a partial glimpse into his nature prevents us from hyper-personalism.
Avoiding hyper-personalism, in my perspective, is vital to the health of the church moving forward in the 21st-century. So many who speak of God as laypersons or skeptics do so with the view that the natural world runs steadily upon its own mechanical course, and God is a supernatural super-person with a white beard watching us from the clouds. This perspective is highly anthropological and sees God as merely an extra (yet somewhat unnecessary) bit added onto the world.
But of course, God is not a white man in the sky. God does not live in the clouds, nor is he confined to any particular location. The world is not wholly independent but rather relies upon God’s constant sustaining energy. A conception of the world that allows God to be ‘give-upable’ is not Christian. Natural theology helps us avoid this.
Finally, natural theology helps ensure robust and reasonable religious discourse. We are language-oriented animals. It is undeniably one of the most significant factors of Homo sapiens. In order for language to be coherent, it must lack self-defeating contradiction. Thus, faith requires reason in order to express itself.
I agree with Barth that faith is a response to God’s self-revelation in the person of Christ, which is a non-propositional revelation. However, we are required by Jesus himself to share the gospel with others. Thus, faith must be communicated through language, which means faith requires reason as a guardian of that language. Natural theology is thus a tool to ensure reasonable religious discourse. However, natural theology must be understood as a TOOL and must be placed underneath revelation.