http://ancienthistory.about.com/od/f...omeReasons.htm
Christianity
When Constantine established religious toleration in the Roman Empire, he took upon himself the title of Pontiff. Although he was not necessarily a Christian himself (he wasn't baptized until he was on his deathbed), he gave Christians privileges and oversaw major Christian religious disputes. He may not have understood how the pagan cults, including those of the emperors, were at odds with the new monotheistic religion, but they were, and in time the old Roman religions lost out. Although the path from pagan to Christian Rome had a few more hurdles, it was from the time of Constantine that Roman Christianity is dated. At this early point, however, the emperors of Rome controlled the religion since emperors held the power to appoint bishops. Over time, Church leaders became influential and took away power from the emperor. Christian beliefs conflicted with the working of empire.
Christian Emperors, Persia, and The Fall of Rome
This is a chapter from Frank Smitha's "Antiquity Online" about the division of the Empire into East and West and the reign, including conversion of Constantine.
Vandals and Religious Controversy
Vandals took over the Roman territory in Africa, just as Rome lost Spain to the Sueves, Alans and Visigoths. A perfect example of how interconnected all the "causes" of Rome's fall are,
Rome lost revenue along with the territory and administrative control. It needed revenue to support its army and it needed its army to keep what territory it still maintained. See: "The Decline of the Roman Power in Western Europe. Some Modern Explanations," by Norman H. Baynes.
The Journal of Roman Studies, Vol. 33, Parts 1 and 2 (1943), pp. 29-35.