Karl Barth was in his early thirties and a country pastor in Safenwil, Switzerland when he wrote the first edition of his landmark commentary The Epistle to the Romans (Der Romerbrief, 1919). Barth did not have any advanced theological degrees when he wrote Romans, and he wrote it while ministering to blue collar workers, who called Barth "comrade pastor", and this is why Barth is often called "The Red Pastor from Safenwil".
Barth was indebted to his teacher Wilhelm Herrman and many liberal Protestant theologians when he wrote Der Romerbrief (1919), and this first edition is commonly known as "Romans I", which was widely and positively received. However, it was Barth's second edition of Der Romerbrief (1921), commonly known as "Romans II", that made Karl Barth world famous, and resulted in Barth leaving his church in Safenwil in 1921 to be appointment the Professor of Reformed Theology at the University of Göttingen, Germany.
Barth's Romans II was a complete rewrite of Romans I, and many Barthian scholars argue that Romans II demarcates Barth's transition from his early period of liberalism while at Safenwil, to his later monumental theological achievement in his Church Dogmatics (1932-1967). Hans Urs von Balthasar famously said, "Between 1922 and 1932, Barth gradually made his way from Romans to Church Dogmatics." 
In this post, I will discuss the significance of Karl Barth's Der Romerbrief.
Romans I (Der Romerbrief, 1919)
It is a travesty, that Karl Barth's first edition of his Romans commentary (Der Romerbrief, 1919) has not been translated into English—the publishers have failed us! Romans I was born out of Karl Barth's experiences as a minister with the Safenwil workers, and from studying under Wilhelm Herrman, that resulted in socialistic and liberal Protestant themes laden throughout Romans I. In lieu of an English translation of Romans I, there are many qualified summaries of Romans I that I recommend, such as Balthasar's summary in The Theology of Karl Barth; Here's a selection from this summary:
Balthasar writes, "What a startling book it is! Barth's opening chords reverberate throughout. It chants of a radical, philosophical mysticism, of a radical historical outlook on the world, and of a powerful universalism deeply tinged with liberalism and socialism.
The Foreword to the second edition of Romans claims that the whole edifice has been razed from the ground up, so that not one stone has been left standing on another. Perhaps Barth should have been more careful and selective in his revisions; could such overpowering themes be done away with so easily?" 
Romans II (Der Romerbrief, 1921)
Karl Barth published six editions of his Epistle to the Romans (Der Romerbrief) between 1919 and 1928, but his second edition was a thorough revision of his first edition Romans I, such that Romans II is considered to be a completely different book, and the remaining four editions are updates to Romans II, and are not radically new revisions like Romans II. So when you purchase Karl Barth's Epistle to the Romans, you are receiving the Romans II edition. Karl Barth's personal assistant and biographer, Eberhard Busch describes the complete re-write of Romans II, in Karl Barth: His Life from Letters and Autobiographical Texts, in this way:
Busch writes, "As a new edition of Romans had become due, Barth resolved to rewrite his interpretation from scratch. He felt oppressed by the 'need to subject the book to a revision in which hardly one stone of the original edifice is left on another.' . . . 'our eldest, who was then a girl of six, told anyone who was prepared to listen that 'Daddy is writing another Romans, much better'. What the angels may have said on this occasion is another question." 
Busch says that Romans II appeared in a summer swelter, rapidly rewritten in eleven months, and completed in the Christmas season of 1918. Theologians, such as Jurgen Moltmann, have noticed that Barth has a two edition pattern, with the first edition being a "false start", followed by a second edition that is the monumental achievement. Romans I is the false start, before Romans II. Another example was Karl Barth's Christian Dogmatics (also not available in English translation), followed by the Church Dogmatics.
Busch writes, "Barth worked with exceptional energy on the second version of his Romans. He wrote the 521-page book in a mere eleven months, sending the pages straight off to the publisher as he finished them. During this period 'my parishioners often had to put up with a pastor who lived in his study'. . . . In August 1921 he said to Thurneysen, 'I shall never forget this hot summer. I amble like a drunken man back and forth between desk, dinner table and bed, travelling every kilometer with my eye already on the next one." 
How is Romans II different than Romans I? Eberhard Busch explains:
Busch writes, "While Barth felt that the first edition of his Romans had been written in a 'still very nebulous and speculative form', the second edition, which came into being page by page between autumn 1920 and summer 1921, presented the reader with 'sharply contoured antitheses'. Between the first and the second editions Barth thought that he had moved 'from Osiander to Luther'. True, 'even now there will be all kinds of oversights and dislocations, but I think that I am a bit nearer to the truth of the matter than before. At any rate, the pantheistic tinge has now been removed. I confess that while I've been lopping off all the luxuriant growths (correcting the first edition) I've been feeling rather like Abraham having to sacrifice Isaac'."
Both Busch and Balthasar believed that Romans II remained to be on the same continuum as Romans I, despite Barth's complete revision (c.f Balthasar's comment on Barth' foreword in Romans II). However, Barth understood his work to be a shift away from the popular liberal Protestant theology of his day. Whether Romans II represents Karl Barth's maturest theology, is subject to debate, but Romans II is a significant event in Karl Barth's life work, that drew the world's attention.
Barth's shift from Liberalism
Most scholars differentiate between the "early Barth" and the "later Barth." The earlier Barth signifies Karl Barth's writings leading up to and including Romans I, and the mature Barth refers to Barth's writings beginning with Romans II, and is best represented by his Church Dogmatics. Barth's transition from the early Barth to the mature Barth wasn't over night, and some scholars suggest that Barth's maturest works do not begin until the Church Dogmatics (or even midway through them, i.e. Vol II/2, pace. Bruce McCormack). In Romans II, Barth believed that he was departing from his former liberalism of his former teachers. Barth's break from Protestant liberalism happened over time, and it wasn't until the 1930's when Barth saw his former teachers and peers sign the Nazi's Aryan Paragraph, that he realized that he could no longer affirm the liberal theology they represented, as he had in his early career, and signifies Barth's famous departure from Liberalism.
Barth's complete revision of Romans I, demonstrates that Barth's theology did change substantially in Romans II. Barth believed he could no longer repeat what he had written before, and he had to say what he said again, but in a new way. The shift is perhaps most significant, because Barth believed he had changed, and was intentionally moving away from his older theological patterns, even if he had not changed entirely as other theologians believe in retrospect.
Eberhard Busch, provides a helpful explanation of Barth's change in self-understanding, and new found theological orientation:
Busch writes, "So now the second edition represented much more clearly than the first the bold attempt to introduce a theology 'which may be better than that of the nineteenth century and the beginning of the twentieth in that it is concerned quite simply with God in his independent sovereignty over against man, and especially the religious man, and seeks to approach God as we believe that we can see him in the Bible.' This attempt contained a radical criticism of the liberal and 'positive' theology of the previous century, arguing that it had ceased to acknowledge God as God." 
Also, Busch provides a helpful summary of Barth's new emphasis in Romans II; similar to Balthasar, Busch gives us insights into the contents of Romans I, by contrasting Romans II to it.
Busch writes, "Barth asserted, shouted, declared, spelt out in a constant variety of new dialectical 'meanderings' that God--is God. To make that clear, he said it above all with an abundance of negative definitions (this is where the second edition of Romans differed from the first): he stressed that God could not be conceived of, that he was beyond this world, wholly other, remote, alien, hidden, that he questioned and indeed negated man and especially his faith, the church and all conceptions of the deity. 'God! We don't know what we are saying. The believer knows our ignorance.' The new world touches the old 'as a tangent touches a circle, that is, without frontier, as the new world.' God's revelation is at the same time the 'most complete veiling of his incomprehensibility.' Accordingly the true character of faith, doctrine, worship, the church, is always merely that of a 'vacuum', a 'crater formed by an explosion', a 'starting place in the air'. So according to Barth God is not an opiate for men, but their limit: he does not restore their equilibrium, but upsets them, confronts them with 'crisis'." 
Forward to the Sixth Edition
Already, I've quoted Balthasar that Barth's Romans II may not be such a radical departure from Romans I after all. Barth re-wrote Der Romerbrief, but theological themes in Romans I were carried forward in Romans II. Barth expressed similar things in new and better ways, and this is why Romans II was a huge success, and led Barth to world-wide fame. Barth had not completely abandoned his old theological paths, but he had not yet begun all of his later theological paths as well.
Four additional editions of Der Romerbrief were published after Romans II, but they were not substantially different than his completely revision in Romans II. Barth's continual developement is exemplified by his comments to the sixth and final edition of Der Romerbrief in 1928. As a conclusion, read the following quotation from the preface to the sixth edition of Der Romerbrief, to see that Romans II is not Barth's final word after all.
Karl Barth writes, "The two years and a half which have passed since the publication of the fifth edition of this book have increased the distance separating me from what I had originally written. Not that, in expounding the Pauline Epistles, or indeed any part of Holy Scripture, I should now wish to say anything material different from what I then said. . . . However, I do not wish the book to go forth once more without saying that, were I to set to work again upon the exposition of the Epistle, and were I determined to repeat the same thing, I should certainly have to express it quite differently. . . . Much would therefore have to be drastically curtailed, and much expanded. A great deal of the scaffolding of the book was due to my own particular situation at the time and also to the general situation." 
Karl Barth's The Epistle to the Romans is an incredibly famous and important theological work, that made Karl Barth world famous and launched his career. Barth's theology in Der Romerbrief (1921), commonly know as Romans II ("romans two") is even more important than Romans I ("romans one"), but both of these books ultimately must be cross referenced against what Barth writes in his magnus opus, the Church Dogmatics, because it does not contain his most mature theological ideas. I haven't fully summarized the content of Romans II, but I have explained its theological significance, such that I hope you feel inspired to read Karl Barth's The Epistle to the Romans.
1. The Center for Barth Studies, biograph, <http://barth.ptsem.edu/karl-barth/biography>
2. Balthasar, Hans Urs von. The Theology of Karl Barth: Exposition and Interpretation, trans. Edward T. Oakes, Communio Books, Ignatius Press, 1992. Print. 73.
3. Ibid. 52.
4. Busch, Eberhard. Karl Barth: His Life from Letters and Autobiographical Texts. W.B. Eerdmans Pub. Co., 1994. Print. 117.
6. Ibid. 118-9
7. Ibid. 119.
9. Barth, Karl. The Epistle to the Romans, trans. Edwyn C Hoskyns, Oxford University Press, 1980, sixth preface.
Related: autobiography, Bruce L. McCormack, Der Romerbrief, Eberhard Busch, Epistle to the Romans, Hans Urs Von Balthasar, Karl Barth, Liberalism, Red pastor, Red Pastor of Safenwil, Romans I, Romans II, Safenwil