Level 1: General Introduction
Level 2: Socialism, Marxists, and Religion
Level 3: History
On the History of Early Christianity - Frederick Engels
A work of Engels' in which he shows how Christianity was originally a primitive communistic struggle against the oppression of the Roman Empire. This work is a must-read, as it will help one understand the revolutionary, communistic origins of Christianity, as opposed to the "Christianity" defended today by the official church and the ultra-reactionaries in the "Religious Right."