<p>The Plan of Saint Gall (c. 820 CE) is the earliest known representation of an ecclesiastical layout, and indeed is the oldest architectural drawings of the christian era. It serves as a rare witness to the geometric organization of medieval monastic complexes. This study systematically investigates the underlying proportional principles structuring its basilica and examines their recurrence in later ecclesiastical architecture. Through the computational analysis of 200 church floor plans built between the ninth and nineteenth centuries, this research identifies a consistent adherence to key proportional relationships of the nave and aisles and the modular structuring of the architectural grid. The remarkable persistence of these proportions across diverse architectural styles and technological advancements suggests an enduring geometric framework, independent of socio-political transformations. By focusing exclusively on geometric and mathematical considerations, rather than engaging in historiographical debates, this study highlights the atemporal role of proportion as a fundamental parameter in Christian architectural design.</p>