Joseph Vogt's fine book "The Decline of Rome" tells eloquently how the fall of the western Roman empire did not mean the fall of the culture and lifestyle of the Roman civilization.
Joseph Vogt does show how the new German rulers had, in fact, been romanized for centuries. He tells also how they adopted new romanized lifestyle, when they moved into the old lands of the Roman empire as their conquerors.

While reading this fine book I was soon struck by the eagerness with which the new German overlords did adopt the Christian religion. This did happen, even if they at first tended to fall for the Arian heresy that was the main competing version of Christianity at that time. However, soon most of them saw the light and adopted the Catholic version of that religion.
Joseph Vogt does not speculate on the reasons why the Germanic kings did adopt Christianity so easily. However, I think that the main reason is that this religion had already been perfected as a tool for ruling and controlling societies in close co-operation of leaders of the Catholic church and Roman emperors.

It is all too easily forgotten that the Roman emperors were present in person in the crucial church councils where the doctrines of the emerging new state church were nailed down. The Roman emperors and popes soon found out the practical benefits of mutual co-operation.
Catholic church became, in fact, the spiritual arm of the Roman empire. It gave new kind of legitimacy for the dictatorial rule of the Roman emperors. In return they gave The Church the opportunity to take over the whole society and to destroy all competing religions, which they promptly also did.

Europe in 476, from Muir's Historical Atlas (1911).

I have written before how this cozy union of feudal masters and the church was a very easy thing to accomplish in the level of ideas. The idea of single autocratic earthly ruler is inherent in the Christian beliefs of a monotheistic single autocratic ruler of the universe.
When you come to think about it, the Christian religion has been spread mainly through conversion of the feudal masters. They did then spoon-feed this religion to their often not-so-eager nations.

The new Germanic conquerors saw very easily the benefits of a religion that did teach absolute loyalty to earthly masters. This religion promised unwavering support for all feudal masters if they just gave The Church the right to rule the minds of their subjects.
All the original Germanic states founded on Roman lands were converted in this way from top down. The same thing happened in later Denmark, Norway, Sweden, Hungary, Poland, Lithuania and many other countries.

The basic formula was always the same. The feudal leaders and despots saw the immense value of the Christian Church as a tool for creating more centralized states. They also soon saw its value as a tool to enhance the position of the kings over the aristocracy.
There was one other thing that struck me when I was reading the "The Decline of Rome". I realized how the Germanic conquerors formed a new parasitic ruling class that fed on the labors of the population of the conquered lands.

They were in a quite similar situation as the later Arabic conquerors. In the name of their newly minted god they conquered vast Christian and Zoroastrian lands. They were for centuries only a small parasitic ruling class that lived on the levies paid by the conquered subjects.
The end-results were, however, quite different. Germans adopted the belief-systems and lifestyle of the conquered, but Arabs did destroy the older cultures. Arabs replaced the conquered old cultures with a new mixture that was built on their own religion and their own language. The difference of course comes from differences in belief-systems.

Germans conquered lands that were already infected with an extremely infectious religion. However, the Arabs were carriers of a new even more infectious religion that had originally developed from Judaism and Christianity. Both of these religions had the ability to take over one's mind completely and even make him or her self-destruct.
This is not very easily accomplished. The will for self-preservation is one of the strongest inherent forces in the human mind and overriding it is always the crowning achievement of any ideology.

(This piece was completely rewritten in 14th of July, 2012)