Americanism, in Roman Catholic church history, a certain set of doctrinal proposals concerning the adaptation of the church to modern civilization that was reprobated by Pope Leo XIII in his apostolic letter Testem Benevolentiae of Jan. 22, 1899. The letter was written in response to a controversy in France following the publication there in 1897 of a translation of a biography of Isaac Thomas Hecker, founder of the missionary Paulist Fathers. The views attributed to Hecker included minimizing certain traditional doctrines likely to be obstacles to conversion.