One scenario could be that the Romans never succeed in conquering Gaul. By the First Century CE, after a century of urbanization and changes in the native political structure of the Gaulish tribes, one of the foremost Gallic confederations (Arverni, Aedui, or the Belgae) unifies the whole country, through warfare or forceful diplomacy, into large, centralized and cohesive civilization that contests with both Rome in the south, and either has designs to expand its territory into Britain and Germania, or dominate them on a cultural and economic level.
The Druid caste of a united Gaul creates or synthesises a new cultural ideology that spreads and becomes accepted by the majority of the Gallic populace as well as the foreign British and Germanic tribal elites whom possesses diplomatic ties with Gaul.
If Christianity still manages to become a dominant ideology in this world as it did in ours, this Gallic-based common culture could become a barrier for the spread of Christianity further north.
Another could be that Arminius, after his successful destruction of the Roman army at Teutoburg, survives the assassination attempts against him by his Germanic compatriots, and manages to forge the western Germanic tribes into a new state. Centuries of Roman cultural influence, steady urbanization, as well as suborning the other Germanic tribes beyond the Rhine or Elbe and forcing them to move into his territory to help expand the populace there, changes them into a formidable kingdom.