Anyone who works on a subject over a period of more than twenty years owes many debts of gratitude. It was in 1971 in his ‘Theological Controversies’ course at Harvard Divinity School that Arthur McGill proposed that we should study the subject of justification on the one hand in Luther, on the other at Trent. I believe that I was immediately captivated. (The second-hand copy of John Dillen-berger's Selections from Luther's writings – which I bought thinking I should only need it for a week – is still with me and in dilapidated condition.) When some years later I came to write a doctoral thesis I had no doubt as to what the topic should be (though I had some difficulty in convincing my teachers). Then there was a day when Arthur McGill asked how Kierkegaard related to all this. I replied, as though it was self-evident, that his was the best solution I had encountered in the history of Western thought to the split between Catholic and Lutheran. ‘There’, he said, ‘is your thesis’.

In the years that I have thought about this topic, first writing a thesis and then more recently this book, many people have talked with me about my work. In 1976 I went to see Philip Watson, whose writing on Luther (at a time when few were interested) remains a landmark. Trained as he was in motif research, he profoundly influenced my own reading of Luther.