Karl Barth on Romans 2:15a:
They show the work of the law, written in their hearts. They come under the judgement of God; they are even now under judgement; and that by which men are justified by God is discovered in them. How, we ask, is this so? The work which is displayed before God by the Gentile who has been justified, and by which he is found pleasing to God, has no positive content or ‘extent’, for this would be irrelevant. Were he to be judged by the righteousness of men, he would undoubtedly be lost. Even if a righteousness of men were possibly discovered in him, he would not thereby be justified. What is pleasing to God comes into being when all human righteousness is gone, irretrievably gone, when men are uncertain and lost, when they have abondoned all ethical and religious illusions, and when they have renounced every hope in this world and in this heaven. Beyond every concrete visible thing, beyond everything in the law of which those who possess it approve—the ‘ethical kernel’, the ‘idealistic background’, the ‘religious feeling’—beyond all that is valued in western European culture—’conduct’, ‘poise’, ‘race’, ‘personality’, ‘delicacy of taste’ , ’spirituality’, ‘force of character’—beyond all these things is set that which men have to lay before God, and which He will render (ii. 6) with eternal life. There may perhaps be no more than a quite unconscious feeling for religion in no way derived from the Church; perhaps no more than the last stage of human nakedness (Dostoevsky!); perhaps no more than confusion, misery, and destitution; perhaps no more than some last terror before the mystery of death, some final disgusted rejection of the inevitability of the world by a man when he leaves his busy life protesting against its futility. But more than any of these, and better and more beautiful, is that the rendering of God—depends upon—nothing at all! There is joy in heaven over one sinner that repenteth, more than over ninety and nine just persons, which need no repentance. But what is Repentance? Not the last and noblest and most refined achievement of the righteousness of God in the service of men; the work of God and not from men, occasions, joy in heaven; that looking forward to God, and to Him only, which is recognized only by God and by God Himself. (Karl Barth, Der Romerbrief, 67-8)
For Barth even the “Law” apart from Christ only becomes an occasion for boasting (think of the Pharisees, my example), it brings a knowledge of God which left to itself is idolatry (read, no knowledge of God). What is required is an in-breaking of God’s life into the human situation, as Barth says: “. . . that the rendering of God —depends upon— nothing at all!
On a related note, Barth’s understanding on a “Natural Knowledge of God,” or Natural Theology, is a resounding Nein! That is, according to passages like Rom. 2:15, and according to Barth’s exegesis, if we allow for man to have natural knowledge of God; God becomes a predicate or “dependent” upon our approach to Him. I think what Barth wants to maintain (per his doctrine of election), and rightly so, is God’s Freedom; which also includes God’s Self-Determined Revelation, wherein God ad extra (or His “economic” disclosure) is God ad intra (His “immanent” or “ontological” being, or who He has been for all eternity). What Barth’s exegesis does is assume that the inner-logic (or presuppositions) of scripture is always going to be proximate to its outer-logic (exegetical reading). Which means that there is a synthetic thread which holds all of scripture together, that is the Revelation of the Eternal Word, or Christ.
Of course as Barth admits, discerning scripture’s inner-logic is disputable; thus the variant readings of its “outer-logic.” I honestly don’t think, at this point, that Barth is off base in his understanding of Romans and Paul. But there is room for me to be convinced otherwise.