Newman's Essay on the Development of Christian Doctrine is one of the most ground-breaking and influential theological works since the Reformation. Written in the lead up to Newman's conversion from the Anglican to the Catholic Church, it traces how early Christianity developed into Catholicism and has been described as doing for theology what Darwin did for biology. It is frequently referenced in debates about how Church teaching develops--what is authentic development and what isn't. "A more intimate apprehension of original dogma" was John Henry Newman's verdict on his Essay on the Development of Christian Dogma. He completed it in 1845 and thirty-three years later thoroughly revised it. The two versions are here compared in a new edition, tracing the process of Newman's developing thoughts--with footnotes and appendices to bring out the importance of this seminal work, which in theological terms ranks alongside The Origin of Species. This is the first critical edition of the Essay. The editor, Newman scholar James Tolhurst, provides detailed notes on the many patristic and historical references in the text which may baffle the modern reader. In an illuminating Introduction he sets the historical context and summarises Newman's idea of development. He also teases out the many revisions which Newman made to his original 1845 text when he revised it in 1878. The result is an edition which makes this classic text accessible both for Newman scholars and for the general reader.