Christianity

General References
Christian Classics Ethereal Library
Internet Ancient History Sourcebook: Christian Origins

New Testament
Bible Gateway

Early Christianity and Nag Hammadi
Guide to Early Church Documents
Nag Hammadi and Berlin Gnostic Library Collection

Catholicism
Catholic Encyclopedia
Vatican: The Holy See
New Advent

Eastern Orthodoxy
TheoLogic Systems Orthodox WorldLinks

Protestant Christianity
Selected Works of Martin Luther (1483-1546)