- The Ecole Initiative
The Ecole Initiative is a “Hypertext Encyclopedia of Early Church History,” extending up to the Reformation. The site is no longer maintained but still provides some useful content: a Glossary containing short (100-200 word) descriptions of several hundred topics; a much shorter index of Articles; and a Chronology. - Internet Medieval Sourcebook
In addition to numerous texts (see below), the Internet Medieval Sourcebook also provides useful indices and supplementary material, such as select Secondary Sources and Maps and Images. - 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. - 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. - 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 (see below) and, of course, medieval Christianity. - 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.
NAPS - The North American Patristics Society
Dedicated to the Study of the History & Theology of Early Christianity