What is this unspoken rule and how does it affect Douglass?

From the text:

"I have said my master found religious sanction for his cruelty. As an example, I will state one of many facts going to prove the charge. I have seen him tie up a lame young woman, and whip her with a heavy cowskin upon her naked shoulders, causing the warm red blood to drip; and, in justification of the bloody deed, he would quote this passage of Scripture—"He that knoweth his master's will, and doeth it not, shall be beaten with many stripes."

I don't know what you mean by unspoken rule?