The essay compares Liverpool Metropolitan Cathedral and Clifton Roman Catholic Cathedral in Bristol to show how uncertainty within the Church opened up a conceptual space that modern architects could fill in various ways. Modern architecture offered aesthetic certainties - in the case of Gibberd at Liverpool, a diagrammatic plan, produced as a result of the competition process, but undermined in execution by an indecisive client committee; at Clifton, a liturgically-ordered plan based on functional study, with a modest exterior that responded to anxieties about the desirability of church building. At a time of continual change in the Church, modern architects provided answers to the problem of what the Church should look like in the modern world. This essay draws on detailed archival evidence from the two cathedrals to explore their processes of design through negotiations between architects and clients.