Biographisch-Bibliographischen Kirchenlexikon (BBKL)
Comprehensive in scope, this encyclopedia includes many entries on both early Christian authors and contemporary German theologians, church historians, and biblical scholars.
ORB: Online Reference Book for Medieval Studies
A larger project of which the Internet Medieval Sourcebook is a part. While ORB contains comparatively little on early Christianity per se, it does have related materials on Late Antiquity and medieval Christianity.
ChurchRodent: Rich Tatum’s Glossary of Christian History
This little hypertext encyclopedia, while not a work of rigorous scholarship, is a lot of fun and fully searchable. ChurchRodent is a product of the site author’s Church History class project for which he wrote a paragraph on every index entry for Bruce Shelley’s Christian History in Plain Language.
New Advent: The Catholic Encyclopedia
This is an online version of the old Catholic Encyclopedia (New York, 1907-12), which contains many articles on early Christian topics.
Internet Medieval Sourcebook
In addition to numerous ancient texts, the Internet Medieval Sourcebook also provides useful indices and supplementary material, such as select Secondary Sources and Maps and Images.