The following reference works are not specifically focused on ancient Christianity or the Mediterranean context in which it emerged and spread, but they also include useful, quality introductions to a range of historical figures and topics in the classical and late antique periods.
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The second edition of a resource that is considered a standard reference in the field. Presents a cross-cultural approach that emphasizes religion's role within everyday life and as a unique experience from culture to culture. The original 2,750 entries have been retained, many heavily updated, and approximately 600 entirely new articles have been added by an international team of scholars and contributors.
Prized for its scholarly presentation of persons and subjects related to Catholicism and the humanities, this encyclopedia features contributions from hundreds of scholars from all over the world, under the guidance of Catholic University of America. Incorporating content from the five supplementary volumes to the first edition of the New Catholic Encyclopedia, this edition also features revised and new articles. Among the 12,000 signed entries in the Encyclopedia are articles on theology, philosophy, history, literary figures, saints, musicians and much more.
Other general reference:
The Book of Saints: A Comprehensive Biographical Dictionary 
This book contains biographical entries for all those who have been formally canonized or beatified by the Roman Catholic Church to date (2015), as well as for those who have had their local veneration approved by the church as a whole. Together with these, entries are provided for those listed in both the old (last edition 1924) and the revised (2004) Roman Martyrologies. The result is claimed to be the only complete dictionary of all the saints listed in both Martyrologies – in any language.
Oxford Dictionary of the Saints
Offers more than 1,700 entries covering the lives, cults, and artistic associations of saints from around the world. Ranging from the famous to the obscure, the rich to the poor, and the academic to the uneducated, the saints profiled here come from all walks of life, from all periods of history, and from around the world, reflecting a wide variety of personalities and achievements.
Stanford Encyclopedia of Philosophy 
Produced by Stanford University, this online encyclopedia includes 1,600 up-to-date entries, and offers valuable introductions and overviews on a wide range of ancient and medieval philosophers across religious traditions: pagan, Christian, Jewish, and Muslim. Each entry has a table of contents and bibliography of recommended readings. Especially useful entries would include: St. Augustine, Boethius, Origen, and Plotinus.
The following reference works are standard for research in the classical context in which Christianity emerged, and in the late antique context in which the Church Fathers flourished. The relevant entries in these encyclopedias and dictionaries, all assembled and written by teams of scholars, are collectively a useful way to gain some familiarity with an historical figure or topic, and current scholarly interpretations and debates.
Brill´s New Pauly is the English edition of the authoritative Der Neue Pauly, published by Verlag J.B. Metzler. The encyclopedic coverage and high academic standard of the work, the interdisciplinary and contemporary approach and clear and accessible presentation have made the New Pauly the unrivaled modern reference work for the ancient world.
Edited by the renowned medievalist Joseph Strayer, this essential reference series contains articles on various aspects of life in the Middle Ages, from AD 500 to 1500 and covers a geographic area including the Latin West, the Slavic world, Asia Minor, the lands of the caliphate in the East, and the Muslim-Christian areas of North Africa. Includes entries of direct relevance to the later patristic authors (i.e. sixth to eighth centuries).
Encyclopedia of Ancient Christianity
Covers eight centuries of the Christian church and comprises 3,220 entries by a team of 266 scholars representing a variety of Christian traditions. It draws upon such fields as archaeology, art and architecture, biography, cultural studies, ecclesiology, geography, history, philosophy, and theology. The coverage extends chronologically from Christianity's origins to Bede (d. 735) in the West and John of Damascus (d. ca. 749) in the Greek East, with detailed emphasis on the first four centuries of Christian history.
Encyclopedia of Early Christianity
The second edition includes about 250 new entries, plus twenty-five new expert contributors. Bibliographies are greatly expanded and updated throughout; More focus on biblical books and philosophical schools, their influence on early Christianity and their use by patristic writers; More information about the Jewish and pagan environment of early Christianity; Greatly enlarged coverage of the eastern expansion of the faith throughout Asia, including persons and literature; More extensive treatment of saints, monasticism, worship practices, and modern scholars; Greater emphasis on social history and more theme articles.
With over 6,700 entries, The Oxford Classical Dictionary is the most authoritative and accessible dictionary of its kind, an essential reference for both scholars and non-specialists with an interest in the Classical era. Includes many entries of relevance to the emergence of Christianity, Late Antiquity, and the Church Fathers.
Oxford Dictionary of Late Antiquity 
The first comprehensive reference book covering every aspect of history, culture, religion, and life in Europe, the Mediterranean, and the Near East (including the Persian Empire and Central Asia) between the mid-3rd and the mid-8th centuries AD. This broad and multi-disciplinary text consists of over 5,000 entries written by nearly 500 contributors and provides succinct and pertinent information on a range of subject areas.
A patrology is a handbook of literature by the Church Fathers, which typically lists editions of their writings in the original languages (mainly Greek and Latin, but increasingly in the Eastern languages), translations into different languages, including English, and an essential bibliography of modern scholarship.
The Clavis Clavium, or "Key of Keys," is the go-to database to access late antique and medieval Christian literature. It incorporates the almost 8,000 pages of data found in four so-called claves: the Clavis Patrum Latinorum and the Bibliotheca Hagiographica Latina, the Clavis Patrum Graecorum and the Bibliotheca Hagiographica Graeca. Together, these four indispensable tools provide researchers working on the extensive field of Early Christian, Medieval and Byzantine literature with essential information regarding authorship, authenticity, chronological and geographical contexts, manuscript transmission, and editorial history. Note that one must search for authors using their Latin names (e.g. "Augustinus," and not "Augustine").
Johannes Quasten's multi-volume Patrology (often simply known as "Quasten") has long been the standard comprehensive listing of the Greek and Latin Fathers. Entries include each Father's works, and references to their critical editions, available translations, and bibliography.