Why does Douglass attack the church, especially given the fact that many abolitionist groups were affiliated with churches

Douglas attacked the church because he believed that the Church would have done something about the issues of slavery if they were not so indifferent about it.

According to him, the church was as wicked and also indifferent about the issues of slavery that the blacks were facing.

Most of the antislavery movements at the time were part of the church, Douglas believed that if the church really cared about the issues that the problem would have been eradicated.

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Explanation:

Douglass attacked the religion, particularly given the fact that several antislavery movements were associated with churches, Douglass opposes the church and if they weren't so evil and insensitive, the church would do something regarding slaves.