Ultimately, after the western part of the Roman Empire fell in the late fifth century, it was the Christian church that carried on at least parts of Roman civilization, learning, and culture. One of the historical ironies of this period of history is that even though Rome's Empire began to decline and (eventually) collapse politically, it lived on thanks to a ideas and beliefs that originally arisen in the Roman context - it lived on ideologically and spiritually.
Image Citations (Wikimedia Commons)
Defeat of Valerian - Ali Ganjei
Divided Empire Map - Creative Commons, historicair
The Tetrarchy - Dennis Jarvis
Mithras - Jastrow
St. Simeon - AxelBoldt