Okay so in the south in the past, not so much anymore, people were extremely religious. For most african american people, and other slaves, God was their only hope at freedom. For Martin Luther King to include biblical references is like a teacher including a football reference in class to gain respect and followers from all the people listening. It gained hope from all slaves. i dont know if this helps, and you may want to expand a bit more if you understand what i am trying to say.