The detail that best supports the interpretation of the correlation between the decline of religion and an increase in human suffering in Arnold's "Dover Beach" is:
Arnold describes the Sea of Faith as retreating, then describes the world “as on a darkling plain / Swept with confused alarms.”
This detail illustrates the idea that as faith diminishes, the world becomes more chaotic and troubled, suggesting a direct link between the loss of religious belief and an increase in human suffering.