Today, apart from taking time to start on Rowan Williams's excellent and demanding Faith in the Public Square, I met an academic friend at the University of Basel and then we went to visit Karl Barth's house and archive.

Barth's house is not marked in any way and I wouldn't have found it by myself. The archivist, Dr Peter Zocher, was very welcoming – and is clearly an expert on the great man. I know this is really cringy, but I found it moving to hold in my hand Karl Barth's copy of Hitler's Mein Kampf and read the margin notes, underlinings, question marks and exclamation marks he had written in pencil. Having replaced that, Peter then opened up a document file and showed me the original handwritten draft of the Barmen Declaration of 1934 – probably one of the most important political and theological documents of the twentieth century.

I have to keep reminding myself that Barth didn't know how the story would end. As Hitler consolidated his power, it took great courage and clarity of mind to challenge him and the violent worldview he was soon to inflict on the whole world. All Barth and his friends had to go on was what they saw and heard at that point, and yet they recognised the evil that was being grown among them. (Barth himself refused to swear the personal oath of allegiance to Hitler, resigned his professorial chair at Bonn, and moved back to Basel.)

I also looked through Barth's personal copy of Dietrich Bonhoeffer's first books – including Nachfolge (translated into English as The Cost of Discipleship). Again, he had marked the text with questions and comments I couldn't decipher.

There is something powerful about holding and reading books that had belonged to, been read by and marked by a giant of twentieth century theology.

However, what I found haunting – and very difficult to put out of my mind – was the discrepancy between the theological sharpness of Barth and his domestic arrangements. Charlotte von Kirschbaum had moved in to the Barth home in Germany and moved with the family (his wife, Nelly, and their five children) to Basel. They lived, effectively, as a ménage-à-trois until von Kirschbaum was elderly and ill. Karl died in 1968, Charlotte in 1975 (after suffering from Alzheimer's for many years), and Nelly was the last to pass away in 1976. All three are buried in the same grave in Basel's main cemetery.

Barth's social skills were not great. Those still alive who remember him relate how difficult it was to know how to handle him. If you wanted to invite him for dinner, who did you invite come with him – Nelly, Charlotte or both? When Barth was away he would write letters to Charlotte, but never to his wife.

One of my abiding questions is how we judge theology in the light of the experience of those who propagate a particular theology. For example, does the fact that Heidegger supported Hitler (which Barth condemned) influence the credibility of his theological perspectives or his philosophical project? Is there a relationship between the nature of Rudolf Bultmann's theology and the fact that he was able to retain his professorial chair under Hitler when other Christians were paying a very high price for their discipleship of Jesus? And, if we are to take this seriously, how does the reality of Barth's domestic relationships impinge on his theology – especially the clarity of his ethical writings?

Or doesn't it matter?

Correction: I had misunderstood a point about Barth's letters. He wrote many letters to his wife, too, but these have not been published. Barth put in his last will and testament that his private correspondence with his wife should not be published.