Development of Christian Doctrine

An Essay on the Development of Christian Doctrine by Bl John Henry Newman, 379 pages

Published in 1845, the year Bl John Henry Newman converted to Catholicism, An Essay on the Development of Christian Doctrine is, in many ways, more of an apologetic work than Apologia Pro Vita Sua, which was written decades later.  The Oxford Movement had attempted to establish Anglicanism as the faithful inheritor of the pure apostolic tradition, avoiding both Protestant errors and Roman accretions, and here the leader of that movement concludes that what he formerly regarded as accretions were, in fact, logical developments, and that such developments must take place in any living institution.

Much of the book is taken up by Newman's explication of the sources of distinctively Catholic* doctrines in the writings of the Church Fathers, demonstrating them as following naturally upon already accepted premises, assimilating thoroughly with the whole.  Of more general interest are the opening chapters, where Newman lays out his principles of faithful development and of corruption, which apply to any institution.  The entirety is masterful, and massively influential - the ideas presented here played a major role at the Second Vatican Council.

* - Where "Catholic" is opposed to "Protestant".  The Orthodox, sadly, were not part of the conversation.