The author traces internal Roman Catholic resistance to the doctrine of papal infallibility, following debates among bishops, clergy, and laity across several countries and centuries and culminating in the Vatican Council’s 1870 decree. Relying chiefly on Roman Catholic sources, private correspondence, and continental scholarship, the account outlines the intellectual, historical, and moral objections advanced, the shifting positions of prominent ecclesiastics, and the practical strains of defining and enforcing the dogma within the Church. Emphasis is placed on how opposition formed internally and on its effects for clerical and lay relations rather than on external critique.