In this article the author develops four essential methodological guidelines, discovered while engaged as a European public theologian in the debate on ageing. They might be exemplary for public theologians working in other spheres of the public domain as well. Dietrich Bonhoeffer's Christological understanding of reality functions as a leading intuition. A public theologian (1) finds her- or himself involved and already situated in discursive practices; (2) critically and constructively contributes to the development of 'social imaginaries'; (3) is an expert in her or his field; and (4) looks for allies and gladly cooperates with them. In this article the author develops four essential methodological guidelines, discovered while engaged as a European public theologian in the debate on ageing. They might be exemplary for public theologians working in other spheres of the public domain as well. Dietrich Bonhoeffer's Christological understanding of reality functions as a leading intuition. A public theologian (1) finds her- or himself involved and already situated in discursive practices; (2) critically and constructively contributes to the development of 'social imaginaries'; (3) is an expert in her or his field; and (4) looks for allies and gladly cooperates with them.