Final answer:
Frederick Douglass provides evidence that Captain Auld justified his cruel treatment of slaves by citing scripture, thus finding religious sanction for his actions. After Auld's conversion to Christianity, he became even more brutal, using his supposedly newfound piety as a cover for his increased cruelty.
Step-by-step explanation:
The evidence that Frederick Douglass offers to support the idea that Captain Auld 'found religious sanction and support for his slaveholding cruelty' is multifaceted. Douglass details that after Auld's conversion to Christianity, he did not become more compassionate; instead, his cruelty intensified. Auld would justify his barbaric actions by quoting scripture, such as the passage stating 'He that knoweth his master's will, and doeth it not, shall be beaten with many stripes'. This supposed religious backing gave Auld and others like him a sanctified pretext for their brutal treatment of enslaved people. Furthermore, Douglass observed that religious slaveholders, under the guise of piety, committed the worst cruelties citing religious mandates as their defense. The narrative describes Auld's behavioral changes post-conversion and highlights the hypocrisy of finding religious motivation for perpetrating inhumane acts upon slaves.