In a recent post, I responded to Richard Muller’s criticism that Karl Barth’s rejection of natural theology as a valid way of obtaining knowledge of God constituted an erroneous and damaging revision of the concept of revelation. I argued, not simply from Barth but, more importantly, on the basis of Scripture, that revelation is reconciliation and that no division should be made between a knowledge of God that leads to salvation and a knowledge of God that does not.
Those who disagree usually make immediate reference to Romans 1:18-32 which, in their minds, deals a fatal blow to Barth’s position. Often, they think that this text is such an obvious defeater that it requires no explanation. Barth, however, was not so naive as to have formed his view in ignorance of this particular passage. In fact, he engages with it at some length in Church Dogmatics I/2, and I find his interpretation extremely illuminating. He writes:
The witness which the apostle declares to the heathen in and with the preaching of Christ, which he therefore awakens in them and makes valid against them, is here emphasised to be their knowledge of God the Creator. The invisible and unapproachable being of God, His everlasting power and divinity, are apprehended and seen in His works from the creation of the world (Rom. 1:20). It is from a knowledge of God, a knowledge of Him on the basis of revelation, that men always start when revelation comes to them in Christ (Rom. 1:19). That is why they can be accused of a “holding of the truth,” a corruptio optimi (Rom. 1:18). We must bear in mind that the very words which are so often regarded as an opening or a summons to every possible kind of natural theology are in reality a constituent part of the apostolic kerygma, whatever contemporary philosophemes may be woven into them.
To bring out the real meaning of the revelation of the righteousness of God in Christ (Rom. 1:17, 3:21), Paul reminds us in Rom. 1:18–3:20 that the same revelation is a revelation of the wrath of God, i.e., that as we are told of the grace which has come to us, we have to perceive and believe our own abandonment to judgment. Grace and judgment are for both Gentile and Jew, both Jew and Gentile, Rom. 1:16, 2:9, and for both Jew and Gentile in the very best that they can do, their worship of God. It is a Christian statement presupposing revelation when in relation to the Jews Paul says that a knowledge of sin comes by the Law (Rom. 3:20). Similarly, it is presupposing the event which took place between God and man in Christ that he says that the knowledge which the Gentiles have of God from the works of creation is the instrument to make them inexcusable and therefore to bring them like the Jews under the judgment and therefore under the grace of God. Here, too, there is no difference. Because Christ was born and died and rose again, there is no such thing as an abstract, self-enclosed and static heathendom. And because Paul has to preach this Christ, he can claim the heathen on the ground that they, too, belong to God and know about God, that God is actually revealed to them, that He has made Himself known to them in the works of creation as God—His eternal power and divinity, which are none other than that of Jesus Christ. Therefore he can tell them that because of their knowledge they are inexcusable before God, if they have “imprisoned” the truth with their ungodliness and unrighteousness.
We cannot isolate what Paul says about the heathen in Rom. 1:19–20 from the context of the apostle’s preaching, from the incarnation of the Word. We cannot understand it as an abstract statement about the heathen as such, or about a revelation which the heathen possess as such. Paul does not know either Jews or Gentiles in themselves and as such, but only as they are placed by the cross of Christ under the promise, but also under the commandment of God. The witness of the hope of Israel, the prophetic revelation, is fulfilled in Christ. By smiting its Messiah on the cross Israel founders on that revelation. It has now become a revelation to both Jews and Gentiles. It now concerns the Gentiles. Therefore the Gentiles have to bow just as emphatically as the Jews to the claim and demand of revelation. Like the Jews, they are addressed on this basis: that from the creation of the world (ἀπὸ κτίσεως κόσμου, Rom. 1:20, i.e., in and with their own existence and that of the whole world)—not of themselves, but by virtue of the divine revelation—men know God, and therefore know that they are indebted to Him. The status of the Gentiles, like that of the Jews, is objectively quite different after the death and the resurrection of Christ. By Christ the Gentiles as well as the Jews are placed under the heavens which declare the glory of God, and the firmament which telleth His handiwork (Ps. 19:2). They are therefore to be claimed as [those who know God] (Rom. 1:21); but only to the extent that, like the Jews, they have not remained such (οὐκ ἐδοκίμασαν τὸν θεὸν ἔχειν ἐν ἐπιγνώσει Rom. 1:28).
It is, therefore, not the case that Paul was in a position to appeal to the Gentiles’ possession of a knowledge of the invisible nature of God as manifested from creation. He could not link up pedagogically with this knowledge. In his proclamation of Jesus Christ he could not let it appear even momentarily that he was speaking of things which were already familiar by virtue of that “primal revelation.” At bottom the Gentiles did not achieve even in the slightest the knowledge of Ps. 19. That is, they did not give God praise and thanks as God (Rom. 1:21). As the sequel shows, this does not mean only a quantitative falling away of their service towards Him nor an imperfection of their relationship to Him. It means rather that the [worship and thanksgiving] which they owe God are not there at all. They have been ousted by another mind and thought and activity which at its root (in negation of the fact that God is revealed to man from the creation) does not have God as its object. “Their thoughts became vain and their foolish heart was darkened” (Rom. 1:21). “They professed (themselves and others) to be wise, and in this they became fools” (Rom. 1:22). And the result was sheer catastrophe: “They changed the glory of the incorruptible God into the likeness of the image of corruptible man, yea of flying and fourfooted beasts and creeping things” (Rom. 1:23). In this idolatry “they exchanged the truth of God for a lie, they worshipped and served the creature instead of the Creator, who is blessed to eternity. Amen” (Rom. 1:25). And in due course the exchange had terrible consequences in the indescribable moral confusion of the human race.
Paul says nothing at all about the heathen maintaining a remnant of the “natural” knowledge of God in spite of this defection. On the contrary, he says unreservedly that the wrath of God has been revealed against this defection: “they which do such things are worthy of death” (Rom. 1:32). Just as revelation had always contradicted heathen religion in the sphere of Israel and on the soil of Palestine, so now, when Jesus Christ has died for all, it contradicts it “publicly,” in its own heathen area, in an apostolic letter which remarkably enough is addressed to the Christians in Rome. There is no such thing now as an undisputed heathendom, a heathendom which is relatively possible, which can be excused. Now that revelation has come and its light has fallen on heathendom, heathen religion is shown to be the very opposite of revelation: a false religion of unbelief.
Although Barth offers his interpretation of Romans 1 in his typically dense style, his overall understanding seems clear. First, he does not deny that creation declares the glory of God (as per Ps. 19), yet he believes, with Paul, that humanity is too blind and deaf in sin to see and hear it. Whatever natural conception of God the Gentiles might have, they transmute into idolatry. Thus, this passage can neither be used to validate the knowledge of God available through natural theology (for this is completely contrary to Paul’s argument!) nor to justify its use by believers (for this is completely beside the point of Paul’s argument which addresses unregenerate humanity).
Second, Barth pays careful attention to the context of Paul’s overall argument. Romans 1:18ff is connected to what precedes with the word “for” (gar in Greek). That is to say, Romans 1:18ff is inextricably linked to what Paul has said previously regarding the gospel that reveals the saving righteousness of God in Christ. The analysis that Paul provides in the latter half of Romans 1 is thus not some timeless truth regarding the human condition; it is rather the divine judgment and verdict pronouned on the non-Jewish world on the basis of the universal lordship of Christ who “was declared to be the Son of God in power according to the Spirit of holiness by his resurrection from the dead”. Paul’s understanding of the plight of all non-Jewish peoples is therefore irreducibly eschatological, that is, it too has only been fully revealed in the revelation of the gospel of Christ: “For in [the gospel] the righteousness of God is revealed…For the wrath of God is revealed against all…” (Rom. 1:17-18). In other words, for Paul, the “natural” knowledge of God in creation that results in humanity’s inexcusability is itself the result of the revelation of Jesus Christ. If we listen attentively to what Paul is actually saying, we will discover that natural theology is not something that stands prior to or independently from the knowledge of God in Christ revealed in the gospel: natural theology is only revealed as it is and for what it is – a corrupt and damning knowledge – in the revelation of the gospel.
Thus it seems that rather than being a defeater of Barth’s rejection of natural theology, Romans 1 would seem rather to fully support it!
 Barth, K., Bromiley, G.W. & Torrance, T.F., 2004. Church dogmatics: The doctrine of the Word of God, Part 2, London; New York: T&T Clark. pp.306-307.