For decades, scholars have assumed that the genius of John Henry Newman remained underappreciated among his Roman Catholic contemporaries. In order to find the true impact of his work, one must therefore look to the century following his death. Newman’s Early Roman Catholic Legacy, 1845-1854 unpicks this claim. Examining a host of overlooked evidence from England and the European continent, C. Michael Shea considers letters, records of conversations, and obscure and unpublished theological exchanges to show how Newman’s 1845 Essay on the Development of Christian Doctrine influenced a host of Catholic teachers, writers, and Church authorities in nineteenth-century Rome and beyond. Shea explores how these individuals employed Newman’s theory of development to argue for the definability of the new dogma of the Immaculate Conception of Mary during the years preceding the doctrine’s definition in 1854. This study traces how the theory of development became a factor in determining the very language that the Roman Catholic Church would use in referring to doctrinal change over time. In this way, Newman’s Early Roman Catholic Legacy, 1845-1854 uncovers a key dimension of Newman’s significance in modern religious history.