How did the two slave masters use the Bible to explain slavery

From the text:

Like William Ford, his brother-in-law, Tanner was in the habit of reading the Bible to his slaves on the Sabbath, but in a somewhat different spirit. He was an impressive commentator on the New Testament. The first Sunday after my coming to the plantation,[Pg 128] he called them together, and began to read the twelfth chapter of Luke. When he came to the 47th verse, he looked deliberately around him, and continued—"And that servant which knew his lord's will,"—here he paused, looking around more deliberately than before, and again proceeded—"which knew his lord's will, and prepared not himself"—here was another pause—"prepared not himself, neither did according to his will, shall be beaten with many stripes."