Complex sentence: The Romano-British population after the withdrawal of the Roman legions remained mostly Christian, although the Anglo-Saxon invaders and settlers who replaced them, founding the English nation, represented a stark return to pre-Christian religion for Britain.
Split into simpler sentences: The Romano-British population after the withdrawal of the Roman legions remained mostly Christian. The Anglo-Saxon invaders and settlers who replaced them, founding the English nation, represented a stark return to pre-Christian religion for Britain.