At long last, Oxford has released this “storied” one volume dictionary now in its venerable third edition. Use this resource to look up most anything unfamiliar to you for a brief, authoritative entry, with a bibliography. Don’t expect anything in depth but libraries have found this work invaluable.. Up to now, Oxford has only made available online a skimpy abridged version (why one would abridge a dictionary is a mystery).
Give it a whirl: Oxford Dictionary of the Christian Church (3rd full edition)
Although maybe a bit over hyped, this publisher blurb does capture its steady if unglamorous utility: “Uniquely authoritative and wide-ranging in its scope, The Oxford Dictionary of the Christian Church is the indispensable reference work on all aspects of the Christian Church.
It contains over 6,000 A-Z entries, and offers unrivalled coverage of all aspects of this vast and often complex subject, from theology; churches and denominations; patristic scholarship; and the bible; to the church calendar and its organization; popes; archbishops; saints; and mystics.”
One useful features of this (or any reliable) dictionary is the quick and dirty search to clear the mind. As one example: I always confuse the Eucharistic doctrinal controversalists of the early middles ages: Ratramnus, Rabanus Maurus, Paschasius Radbertus. Who? Short of having a copy at hand , this online version is a life saver. I just log in and look up a principal in the disputes: See the entry.