Fall of the Roman Empire: Why did it happen?

A Jewish convert to the new religion Christianity named Paul had was most responsible for shaping the beliefs early Christianity. A Christian Jew, Paul also had Roman citizenship therefore he was able to leave the land of Israel and spread the ideas and beliefs of Christianity throughout the Empire. Soon, Christian communities become established in many of the large cities in the Roman Empire. While the vast majority of Jews continued to follow the teachings of the Torah and practice Judaism in Israel, Christianity grows throughout the Empire.
The polytheist emperors of Rome worried that Christianity, which taught pacifism, would turn the emperor weak. The Roman Empire's leaders at first persecuted Christians out of fear. Later, under the Constantine in 321 the Roman Emperor gave up fighting against the Christians and adopts the religion as the official religion throughout the Empire. The Christians established the Catholic Church in Rome. All later Christian churches will eventually break off from this Church. Soon after Christianity is made the official religion of the Empire, the Roman Empire is destroyed by outside nations, by the Germanic tribes from the North. Historians have debated why the Roman Empire was destroyed for generations.
you will read some of the most logical reasons bellow.

possible reason for the fall of the Empire
http://killeenroos.com/1/Romefall.htm