I highly recommend Karl Barth’s slim volume God in Action as a great introduction to his work. Here is a review I wrote for Willie Jennings's Theology of Karl Barth course.
The slim volume God in Action contains five addresses Barth gave between April 10 and September 12, 1934. The pressure of the Nazi government on the churches in Germany during this period was fierce. The Barmen Declaration—written mostly by Barth—was adopted during May of 1934. Repeatedly in God in Action Barth refers to the “remarkable apostasy of the Church to nationalism.” Each of the five lectures attempt to pry off Nazi fingers from the Church: (1) Revelation, (2) the Church, (3) Theology, (4) the Ministry and (5) Witness are only rightly conceived as primarily what God has done and does. Barth urges the Church to take its orders from God rather than human authorities. The title of the work comes from his statement, “What is done to us, God in action for us, is a divine miracle.”
The question for readers of Barth is how to understand his most extreme statements about the dangers of human effort in our cultural contexts which are seemingly not as seriously compromised as his.
Rowan Williams notes that most Christians do not find themselves in imminent danger of martyrdom—they live in “a post-martyrdom period.” He argues that Nazi Germany was a special case.
And in the last century or so, it is significant that believers have from time to time had to confront just such pressure when the alliance of political power and a kind of religious mythology recreates something of the atmosphere of the Roman empire. Thus when in 1936 the Confessing Church in Germany, the network of those who resisted the anti-Semitic legislation of the Third Reich, bound itself to the ‘Barmen Declaration,’ affirming the sovereignty of God in Christ over all other claims to authority, the primitive shape of Christian self-definition became visible once more.
He argues that the Nazi situation constituted “apostasy” whereas in South Africa when apartheid reigned, the church was infected with “heresy.” Williams writes, “The DRC’s acceptance of apartheid was seen as heresy rather than an apostasy; the German Church struggle was more serious, affecting the Church’s liberty to define itself.” Williams is not here referring to Barth. Williams argues that martyrdom is the best way to test the church’s faithfulness. He is simply making the point that martyrdom will look different in different contexts.
However, Barth is quite adamant that the lessons of Nazi Germany do apply to other settings.
My dear friends from England and America, I am from Germany. There we have reached the end of the road at whose beginning you are standing. If you begin to take the pious man serious [sic], if you do not care to be one-sided, you will reach the same end before which the official German Church stands today.
Because of the horrific results of Nazi Germany, no one should dismiss Barth’s comments lightly. But one can also argue that the colossus of Nazi Germany was not solely the result of an emphasis on Pietism or “the Christian life.” Too great a suspicion of Christian human response to the grace of God could encourage a certain Quietism, Gnosticism, Deism, Docetism, or Fatalism that is suspicious of evangelism.
But those who read Barth this way—even the early Barth such as God in Action and The Epistle to the Romans—read him selectively and wrongly. Take for example, a typical statement by Barth: “A witness does not come with the claim, I have something to say. Surely he has something to say. But what he says can only be a reminder of what God has said and wants to have said.” Does Barth then mean that “one need only read the Bible aloud and people will be converted—the word of God does not come back void” as a professor once told me Barth was saying? I would retort, “Is that what Barth does? No!” Barth tries to compellingly do what he says the preacher should do, “the preacher dares, today, to think the thoughts of the biblical witnesses after them, and, in the name of the present-day Church, to speak them as out of his own knowledge.” If one tries to gather quotes from Barth that seem to imply Quietism—a lack of human action—in each case one can read further and see that Barth affirms appropriate human response to the grace of God—while he is indeed adamant to retain the proper ordering. “Have you been told something before you go and say something to others?”
Barth’s God in Action then is a beautiful piece of work which introduces English readers to the tension-filled environment of 1934 Germany under the Nazi Reich and to a theology strong enough to resist it. This neglected little volume deserves to be set alongside and distributed with Martin Luther King’s Letter from a Birmingham Jail and Dietrich Bonhoeffer’s Discipleship—as faithful 20th century Christian responses to the subtle evils of political rhetoric masked in Christian guise. Those who would take it and use it to promote declining attendance, stifling bureaucracy and criticism of evangelism, misunderstand 1934 Nazi Germany and grossly distort Barth’s own message by pulling quotations out of larger nuanced sections. This is a stirring book which urges the church to be the church, to be attentive to the Scriptures, and to hold to them courageously. I will end this review the way Barth ends the book, “it is necessary that a sanctuary be built in the midst of our world. And this sanctuary must not be a hybrid of Church and world, it must be truly Church, a Church which will remind men of the eternal kingdom of God.”
 Karl Barth, God in Action (Eugene, OR: Wipf and Stock, 1936).
 “The Confessional Synod of the German Evangelical Church met in Barmen, May 29-31, 1934.” Karl Barth, “Theological Declaration of Barmen” in The Church's Confession Under Hitler (ed. Arthur C. Cochrane; Philadelphia: Westminster Press, 1962), 237-242. Online: http://www.sacred-texts.com/chr/barmen.htm
 Barth, God in Action, 137.
 Barth, God in Action, 122.
 Rowan Williams, Why Study the Past? The Quest for the Historical Church (London: Darton, Longman and Todd Ltd, 2005), 53.
 Williams, Why, 54. Italics mine.
 Williams, Why, 85.
 Barth, God in Action, 137.
 Barth, God in Action, 136-139.
 Barth, God in Action, 107.
 Barth, God in Action, 53.
 Barth, God in Action, 105.
 Barth, God in Action, 143.
I also wrote an Amazon.com review of one of Barth’s other slim volumes.
Read a "book" by Karl Barth in a couple hours, January 31, 2009
This is a short book by Karl Barth. Actually it is a lecture from October 9, 1929. There are some great moments and gives you a look at his theology. It is 70 pages but almost half of that is endnotes which you can skip. I read it for Willie Jennings's Duke Divinity School course on Theology of Karl Barth (Spring 2009). If you want to read a book by Barth is a couple of hours, this might be the one. I would probably recommend God in Action: Theological Addresses as a slightly more accessible but larger slim volume by Barth.
In the first section, he makes the points: the Holy Spirit is not synonymous with the human spirit (3) and we need to be very cautious before saying "Thus saith the Lord" (10). In the second section, he argues against a view that good works are synonymous with God's grace--only God judges what can be called "Christian" (37). The third section deals with the importance of the eschatological.
On a theological level, Barth takes on Augustine and a Roman Catholic understanding of works in this book so if that is of interest, this would be a good thing to read.