These academic encyclopedias and lexicons should form the foundation of your research process. After consulting these you can move on to primary texts, academic journals, and other secondary literature.
Religion Past & Present : Encyclopedia of Theology and Religion by
This fourteen volume set is a key starting place for research.
Encyclopedia of Ancient Christianity by
A new and updated academic encyclopedia of early Christianity. While it does not displace older works, it is one of the first that you should consult in your research.
Encyclopedia of the Early Church by
This is a classic academic reference work for the first stages in serious research.
Encyclopedia of Early Christianity by 
Theologische Realenzyklopaedie by
An encyclopedia written in German that seeks to represent theological and religious research. It is very helpful when researching individual Church Fathers or themes within early Christianity.
Biblia patristica : index des citations et allusions bibliques dans la littérature patristique by
This work arranges the biblical verses in canonical order and lists the early Christian and patristic literature where the corresponding book and verse is discussed. The various volumes are sorted by specific Church Fathers.
Dictionary of sects, heresies, ecclesiastical parties, and schools of religious thought. by
A dated work, but still a good place to begin research on various sects within early Christianity.
Index Patristicus; sive, Clavis Patrum Apostolicorum operum, ex editione minore Gebhardt Harnach Zahn lectionibus editionum minorum Funk et Lightfoot admissis. by
This work lists Greek words and where they occur within early Christian and later patristic literature.
The Westminster Handbook to Patristic Theology by 
Dictionary of Early Christian Literature by 
The Westminster Handbook to Origen by
This is a thematic introduction to the works and thoughts of Origen of Alexandria.
The Brill Dictionary of Gregory of Nyssa by
This is a dictionary of Gregory of Nyssa's important words and themes found throughout his works.
The Oxford Guide to the Historical Reception of Augustine by
This three volume work is a ground-breaking international and interdisciplinary enterprise on the impact of the thought and work of Augustine of Hippo (AD 354 - 430), arguably the most influential theologian of the early Church.
A Patristic Greek Lexicon by
This is the quintessential Greek lexicon for studying the early Church Fathers.
A Greek-English Lexicon by
This Greek lexicon is a necessity along with Lampe's Patristic Greek lexicon. Lampe's work does not include the Greek words that are found in Liddell and Scott's lexicon, thus making this an indispensable companion alongside Lampe's lexicon.
A Greek-English Lexicon of the New Testament and Other Early Christian Literature by
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Greek lexicon of the Roman and Byzantine periods : (from B. C. 146 to A. D. 1100) by
This lexicon is helpful for later patristic Greek authors (5th-8th century AD).
Oxford Latin DictionaryLexicon Athanasianum. by
This is a Greek lexicon corresponding to Athanasius of Alexandria's works. The definition of the Greek terms are translated into Latin.
