I would hazard a guess…
The encyclical Humanae Vitae, which not only forbade as grave sins the pill and all mechanical means of contraception but also the withdrawal method to avoid pregnancy, was universally regarded as an incredible challenge. Invoking the infallibility of papal, respectively episcopal teaching, the pope pitted himself against the entire civilised world. This alarmed me as a Catholic theologian. I had by then been professor of theology at the Catholic theological faculty of Tübingen University for eight years. Of course, formal protests and substantive objections were important, but had the time not now come to examine this claim to the infallibility of papal teaching in principle? I was convinced that theology — or, to be more precise, critical fundamental theological research — was called for. In 1970, I put the subject up for discussion in my book Infallible?: An Inquiry. I could not have foreseen at the time that this book and with it the problem of infallibility would crucially affect my personal destiny and would present theology and the church with key challenges. In the 1970s, my life and my work were more than ever intertwined with theology and the church.