Karl Barth and Emil Brunner is the greatest tragic love story since Romeo and Juliet. The friendship between Barth and Brunner is nearly as famous as its sudden demise. Brunner's famous essay Natural Theology was responded to by the loudest "Nein!" by Karl Barth that the known world has heard. It is as loud as it was surprising. Does there exist a stronger objection to "Natural Theology" than Karl Barth's 'Nein'?
Before the letter, let me provide some context to the Karl Barth's debate with Emil Brunner on Natural Theology by providing a definition of Natural Theology,
"Natural Theology is the doctrine of a union of humanity with God existing outside God's revelation in Jesus Christ" (CD II/1:168). It is "a theology which grounds itself on a knowability of God distinct from the grace of God, i.e., on a knowability of another God than Him knowable only in His grace" (143). It is "a science of God . . . constructed independently of all historical religions and religious bodies as a strict natural science like chemistry and astronomy 'without reference to or reliance upon any supposed special expectional or so-called miraculous revelation" (KGSG, 3). Barth argued in his famous debate with Emil Brunner (1934) that, strictly speaking, it is "every (positive or negative) formulation of a system which claims to be theological, i.e. to interpret divine revelation, whose subject, however, differed fundamentally from the revelation in Jesus Christ and whose method therefore differs equally from the exposition of Holy Scripture" (Nth, 74-75)
The Westminster Handbook to Karl Barth, ed. Richard E. Burnett, pg 152.
Karl Barth's rejection of Natural Theology is thoroughly worked out his Church Dogmatics (CD II/1). It was a loud No that left no doubt that Barth was firmly against Natural Theology. However, Barth revisited Natural Theology at the end of the Church Dogmatics in CD IV/3.1's discussion of "secular parables of the truth", which was basically a commentary on Calvin's writings in the Institutes of the Christian Religion on this topic. In Barth's writings on "Secular Parables", many have believed that Barth had changed his mind on Natural Theology and opened a door to a secret and hidden Yes behind the Nein. Whether this is true, will be left as an exercise for the reader.
In this letter, that I've quoted in entirely below, is the last correspondence of Barth and Brunner. Barth sent the following letter, that may or may not have been received by Brunner on his death bed. This letter raises many provocative questions: Did Brunner receive Barth's letter? Was this a final reconciliation between theological giants? Did Barth concede and here open the door to Natural Theology in the end? Or were these merely kind words, admitting to the guilt of destroying a friendship? So many questions left unanswered.
Your letter of the second touched me greatly--also because you wrote it.
If I were more active after my two-year illness I would take the next train to press Emil Brunner's hand again.
If he is still alive and it is possible, tell him I commend him to our God. And tell him the time when I thought I should say No to him is long since past, and we all live only by the fact that a great and merciful God speaks hi gracious Yes to all of us.
With sincere thanks and greetings,
P.S. Please give my greetings to Mrs. Brunner too.
 Vogelsanger, after a visit to Emil Brunner's sick-bed, sent a long letter to Barth on his serious condition, 2 April 1966
 According to Vogelsanger, his hitherto good relation with Barth had been disrupted after the founding of the paper Reformatio, which Vogelsanger edited and whose policy Barth did not approve of.
 Vogelsanger received Barth's letter on the morning of 5 April, rushed to the hospital, where Brunner was weak but alive and conscious, and read the letter with Barth's greeting. A slight but beautiful smile came over Brunner's features and he quietly pressed Vogelsanger's hand. A few minutes later Brunner went into a coma from which he did not awake, dying peacefully near midday on 6 April. Barth's seems to have been his last earthly greeting.
Karl Barth Letters: 1961-1968, ed. Geoffrey W. Bromiley, Letter #207
Related: Church Dogmatics, Emil Brunner, institutes of the christian religion, John Calvin, Karl Barth, Natural Revelation, Natural Theology, Peter Vogelsanger, Secular Parables