what is the significance of the opening line in the novel color purple?

The first line of the novel is the only line of direct speech and the only line which falls outside the framework of the letters written by Celie and Nettie.

The novel begins with a 14 year old Celie quoting who she believes to be hear father, "You better not never tell nobody but God.” Unspeakable things have been done to her by this man and the novel takes off from this vantage point into a series of letters. We know they are not going to be happy.