Final answer:
After the Romans withdrew, the remaining Celts faced invasions from Germanic tribes and were integrated into new emerging kingdoms, with Christianity and the Roman Catholic Church influencing the new societal structure.
Step-by-step explanation:
After the Romans withdrew from their territories, the fate of the remaining Celts was varied across different regions. Following the Roman expansion and the subsequent assimilation of the Celts into Roman society, they had adopted Roman customs, language, and governance. However, the collapse of Roman authority brought significant changes.
In territories like Gaul (modern-day France), Celtic tribes encountered invasions from Germanic tribes, such as the Visigoths and the Vandals. The Roman military, having lost much of its previous dominance, could not provide the same protection as before. As a result, the power vacuum left by the Romans led to an era of instability and migrations that transformed the political landscape of Western Europe.
Many Celts would have been absorbed into the emergent Germanic kingdoms, adapting to new rulers and cultural influences. Over time, the development of the feudal system and the rise of Christianity, specifically the Roman Catholic Church, contributed to the cultural and political evolution of post-Roman Europe. The hegemony of the Celts, as they once knew it, substantially diminished, making way for a new medieval order.