Edward Gibbon's six-volume The History of the Decline and Fall of the Roman Empire is a classic in the truest sense of the word. The first volume of this monumental work was published in 1776, with the remaining volumes following over the next 12 years, but Gibbon actually spent 26 years working on Decline and Fall. Gibbon's careful scholarship and exhaustive research have survived the test of time remarkably well. Modern historians frequently cite Gibbon's work as a primary source of information and scholarship for this important period of history.
You've probably heard or read a summary of the reasons Gibbon gives for the decline and fall of Rome (this one is from the Encyclopedia of 7700 Illustrations ):
Many preachers and teachers over the years have drawn parallels between Rome and various modern societies, gleaning lessons from the great empire's fall and warning us that we may share the same fate. Take this opportunity to purchase Gibbon's magnum opus and read for yourself the how and why of Rome's destruction!
In Chs. 15 and 16, and throughout his great work, Gibbon dwells on the outside, and on the defects rather than the virtues of ecclesiastical Christianity, without entering into the heart of spiritual Christianity which continued beating through all ages; but for fullness and general accuracy of information and artistic representation his work is still unsurpassed.
— History of the Christian church , 1997.