I have read histories that tell of St. Boniface and other Anglo-Saxon and Irish monks working in the Rhine valley among Germanic tribes converting them to Christianity. I understand that this work was supported by both the pope and the Frankish kings, most notably Charlemagne who viewed their efforts as complementary to his military battles in extending his empire and influence.
What I do not understand is why the pagan Germanic chieftains wanted any part of this new religion. They had their established system (Woden, sacred oaks, etc). Why did they buy into Boniface and others' claims? What arguments did the monks make? Were there geopolitical rather than religious reasons that pushed them toward Christianity? I understand that earlier people like Visigoths and Ostrogoths had an admiration for the Roman civilization and wanted to participate in it, but that makes much less sense in the 6th and 7th century than in the 4th. I can understand why certain classes in Roman society adopted Christianity, but to the Saxons, Frisians, and Thuringians, who did not settle in Roman-Christian areas, wouldn't it be the religion of the enemy (the eastward encroaching Franks)?