Exactly which Germanic kingdoms do you mean? The Frisians did not convert until after 754 AD (when they killed Saint Boniface - Bonifatius - after he managed to upset them, probably doing stuff like cutting down their holy oak).

The Frisians were never properly incorporated into the Roman empire to begin with, just like many other northern Germanic kingdoms.

As for the EU having a credibility gap, we're doing just fine, thank you very much.