Why does John teach sarny how to read and write in English

As their friendship deepens, John reveals that he was once an escaped slave who made it all the way to the North. There, he learned to read and write before returning to the South to teach other slaves the importance of literacy: “They have to read and write,” he said. “We all have to read and write so we can write about this – what they doing to us. It has to be written” (Page 58). Even after Waller discovers John teaching Sarny to read, and cutting off his toes, John – who by this time has earned the nickname “Nightjohn” for his late-night lessons – does not give up his mission. He successfully escapes to the North from Waller’s plantation and returns in the evenings to continue teaching Sarny and many other slaves how to read.