In speaking that sentence, Frederick Douglass meant that he had found the answer to keeping something as dark, mysterious, and malefic as slavery in society.
We can arrive at this answer because:
Douglass sees this moment as "the revelation, explaining dark and mysterious things," because at that moment he understood the mystery of why the maintenance of something as terrible and dark as slavery happened. He realized that it was because black people did not have access to education unlike whites, so they did not have access to information and critical thinking that would show them how to rebel against the condition they lived.
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