The Germanic tribes that conquered the Western Roman Empire adopted Christianism, therefore abandoning their own Pagan beliefs. In some cases they even started to speak Latin. I don't know if this case is unique in history, but it seems that conquerors are more tending to impose their own religion and language to the conquered. For example, Spaniards imposed Christianism and Castilian to American indigenous, and also to their African slaves who usually were Muslims or Animists. The Turks imposed Islam when they conquered the Byzantine Empire.
I once even read that, had Attila conquered the Western Empire, it could've been the end of Christianism. What made the Germanic different to the Huns, as we are able to suppose that they wouldn't have adopted Christianism? What made them different to the Arabs conquering North Africa or the Turks conquering the Eastern Empire and not adopting Christianism (imposing Islam instead)?
Hi, I've answered this question before, most recently here.
TL/DR: The various Pagan Germanic peoples who settled former Imperial territories did bring their own religions with them, but then converted at a later date. As a literate religion with a highly structured and easily replicable system of governance, bureaucracy, laws and record keeping - not to mention its inherent Romanitas - Christianity was an extremely attractive choice for an early medieval monarch looking to strengthen and consolidate their hold on power, increase their stature and extend their authority. Islam, as another Abrahamic, literate faith, already had these advantages and as such conversion held less appeal.