In his later career, Martin Luther often spoke of the University of Paris as the "mother of all errors". In his early years, however, and even in the first years of his reform, he spoke of the same university with high esteem and named her the "most Christian mother of all universities". The article traces this radical shift in Luther's opinion of the Sorbonne back to the events between 1518 and 1521, events that culminated in the Parisian Determinatio against Luther which destroyed his last hope to find in this institution a conciliarist ally against the Pope. Luther's depreciation of the Sorbonne as the mother of all errors illustrates thus his late struggle for a reform of the whole Latin Church, but it is also illuminating with regard to his intellectual involvement in specific traditions of Late Medieval academic culture.